The tunable mechanisms of lateral heterostructures on both homogeneous junctions and heterogeneous junctions are summarized.Electronic and photoelectronic devices with lateral heterostructures have been discussed.Different types of contacts of 2D lateral heterostructures are classified.Recent developments in synthesis and nanofabrication technologies of 2D lateral heterostructures are reviewed. Recent developments in synthesis and nanofabrication technologies offer the tantalizing prospect of realizing various applications from two-dimensional (2D) materials. A revolutionary development is to flexibly construct many different kinds of heterostructures with a diversity of 2D materials. These 2D heterostructures play an important role in semiconductor and condensed matter physics studies and are promising candidates for new device designs in the fields of integrated circuits and quantum sciences. Theoretical and experimental studies have focused on both vertical and lateral 2D heterostructures; the lateral heterostructures are considered to be easier for planner integration and exhibit unique electronic and photoelectronic properties. In this review, we give a summary of the properties of lateral heterostructures with homogeneous junction and heterogeneous junction, where the homogeneous junctions have the same host materials and the heterogeneous junctions are combined with different materials. Afterward, we discuss the applications and experimental synthesis of lateral 2D heterostructures. Moreover, a perspective on lateral 2D heterostructures is given at the end.  The graphene monolayer, which was successfully prepared through micromechanical cleavage in 2004, triggered enormous research interests because of its exotic physical properties, such as high charge carrier mobility, high mechanical strength, high thermal conductivity, and broadband optical absorption –4. More 2–WS2 heterojunctions, high-mobility field-effect transistors (FETs), p–n diodes with large rectification behavior, superior photovoltaic devices (large open-circuit voltage of 0.47 V and short-circuit current of 1.2 nA), and CMOS inverter with large current gain (as large as 24) have been demonstrated , and having a similar structure to that of 1H–MX2, a 1S–WS2 and 1S–MoSe2 lateral monolayer heterostructure has been built, as shown in Fig. 2 LHSs provide opportunities in new device physics and optical application [2–(MX2)n  LHSs have been studied with first-principal calculations, as shown in Fig. 2 region, where MX2 represents MoSe2, WS2, or WSe2. The CBM states and VBM states of MoS2-(WS2)n lateral heterostructures are contributed mainly by the MoS2 region and (WS2)n region, respectively. This property is related to the width n. When the width n > 2, the MoS2–(MX2)n lateral heterostructures show type-II band alignment. The free electrons and holes are separated in the MoS2 and MX2 regions, respectively, which induce the long electrons’ and holes’ lifetimes. The width-tuned heterostructure provides desired electronic, optical, and photocatalytic properties. The 2D WS2/WSe2/MoS2 in-plane structures are also discussed [Lateral TMDCs 2D heterostructures have been researched a lot because of their unique electronic properties, especially potential type-II band alignment and the wide distribution from semimetal to semiconductor. The electronic properties and inner mechanisms of more and more TMDCs LHSs have been discussed. The 2D square TMDCs . With the bandgaps and band offset tuned by the uniaxial tensile strain, the light conversion efficiency of the structure can be elevated [In addition, the strain can modify the electronic structures and band offsets. With the strain-tuned transition of type-I and type-II band alignment, LHSs have potential application in light-emitting and photovoltaics. Uniaxial tensile strain can modulate the electronic and optical properties of TMDCs LHSs, MoXelevated .2–WSe2 and MoS2–WS2, have been investigated. The interface of LHSs plays an important role in device properties. It is found that these interfaces behave as 1D state with noticeable features [2 and HfS2 nanoribbons, have also been used to fabricate heterostructures [2P2 LHSs behave as quasi-type-II indirect semiconductors. AsmPn  LHSs behave as type-II direct semiconductors, with the VBM localized around arsenene and the CBM localized around blue phosphorene. The electronic properties can change from indirect bandgap to direct bandgap as the width increases. Heterostructures of different component ratios have been discussed. For heterostructures of AsmPm20- , when m < 10, the CBM shifts downward while the VBM shifts upward. When m > 10, the CBM shifts upward while the VBM shifts downward. With increasing width, the bandgap first decreases and then increases. The tensile strain will not change the direct bandgap property. However, the strain has a strong effect on the CBM and VBM states. When the applied strains exceed 6%, the band alignment transition from type II to type I can be observed. The tunable heterostructures have type-II band alignment and high carrier mobility, which can potentially be used in photovoltaic, optoelectronic, and photocatalytic devices.The interface of heterogeneous junctions is the current research focus. The lateral interfaces, both zigzag and armchair, of monolayer TMDCs, such as MoSefeatures . Of courructures . The AlNructures . The staructures . Its banGapless graphene can easily form M/S contact with other 2D materials, such as BN, phosphorene, and TMDCs. The electronic properties of LHSs are highly related to the M/S interface. Lateral graphene/BN heterostructures with different kinds of interfaces, armchair or zigzag, have been studied . The int2 have been studied [2 become more metallic because of the gap state from the interface. The difference potential of the four contact geometries implies that the interface plays an important role in carrier transport of graphene/MoS2 junctions. The structure preference with C–S or C–Mo on growth condition, the charge transfer, and the mid-gap states of the boundary in the lateral graphene–MoS2 interface have also been discussed [The electronic transport properties of four different edge contacts  between graphene and MoS studied . The MoSiscussed . These pI–V curves obtained in zero gate, which implies tunable transport properties under different biases. The calculated mobilities of the two devices are ~ 1700 and 520 cm2 v−1 s−1, respectively, which exhibit good field-effect mobility. Moreover, such devices have good plane integration properties. With this kind of heterostructure, flat electrically isolated graphene devices can be fabricated [I–V characteristics are shown in Fig. Electronic devices based on lateral heterostructures, such as FETs –123, resbricated . Monolay2 as channel, have an on/off ratio of ~ 109 and maximum transconductance gm of ~ 6 µs [gm) curves are shown in Fig. 2, which is shown in Fig. 2–metal heterostructures FETs, the MoS2–graphene FETs have a lower Schottky barrier [Rsquare) in Id–Vds at different temperatures is plotted in Fig. Id–Vds trends in MoS2–graphene and MoS2–metal FETs, the inset is plotted at a temperature of 270 K. Compared with MoS2–graphene FETs, MoS2–metal FETs have larger nonlinearity in the Id–Vds curves, which means the presence of a larger barrier. The Schottky barrier extracted from Arrhenius measurements in different Vg shows that the MoS2–metal heterostructures FETs have a larger barrier, as shown in Fig. 2–graphene heterostructures also have lower noise than the MoS2–metal devices, because of the lower barrier in the contact.Lateral graphene–TMDC heterostructures exhibit some good electronic properties. Top-gate FETs with graphene as source and drain and with MoSf ~ 6 µs . The tra barrier . Linear 2 and WS2 FETs have been built [2 and WS2 lateral heterostructure is etched as a ribbon, and then Cr/Au is deposited on the ribbon as electrons. The optical image is shown in Fig. n-type behavior. The device has a large on–off ratio of 105. The output characteristic (Ids–Vds) is shown in Fig. Ids decreases with the Vds from 80 to 30 V. From the output characteristic, a current rectification behavior can be observed. Although the calculated field-effect mobility is 0.14 cm2 V−1 s−1, the electronic properties can be improved in the future. The devices are hopeful future integrated electric devices and integrated circuit applications.Based on TMDC–TMDC heterostructures, NbSen built . The tri2–graphene photodetecting transistors exhibit photoresponsivity reaching up to 121 A W−1 under 2.7 × 105 mW cm−2 illumination, as shown in Fig. Vg = 30 V) and off (Vg = 0 V) states is given under different laser powers [2 contact. The band diagram of graphene–WSe2–graphene heterostructures is depicted in Fig. 2 heterostructures, the photodetector has a specific detectivity D* of up to 1.4 × 1014, which is of importance in future applications [The optoelectronic properties of lateral heterostructure devices have been investigated intensively. Some optoelectronic devices exhibit superior optical performance –130. In r powers . In Fig.ications .Fig. 6Ph2–WS2 heterojunction photodiode [p-type behavior of the materials. The electronic transfer curves under different illumination powers ranging from 0 to 0.59 mW in Fig. 2 to WS2. A forward bias can easily shift the electrons, but a reverse bias cannot always shift the electrons in the reverse direction. The highest photoresponsivity of this device is 6.5 A W−1 in a light illumination of 0.32 µW at a wavelength of 532 nm. The external quantum efficiency (EQE) of this heterostructure can also reach up to 15.2%. Above results confirm that WSe2–WS2 heterostructures are proper elements for high-performance optoelectronic devices.Because of their desirable type-II band alignment, TMDC–TMDC LHSs have attracted a lot of attention. The type-II band alignment promotes the separation of electrons and holes, leads to the long lifetime of photoexcited charge carriers, and reduces the recombination of electrons and holes at the surface. Heterostructures with type-II band alignment is suitable for optoelectronic device design. The schematic and optical image of a lateral doped WSeotodiode  built byIn order to obtain a brief insight on LHSs, we list typical LHSs in Table The excellent properties of the LHSs discussed above inspire more works about the synthesis of LHSs. It is expected that the synthesis of LHSs can promote more device applications. In this section, we review the experimental synthesis of the LHSs.Although the previous proposed LHS devices have great performance, their applications are hindered by the synthesis technology. Recently, with the developments in synthesis technology, many LHS devices, such as graphene heterostructures and graphene devices, have been successfully fabricated. Graphene nanoribbon heterojunctions were synthesized with a bottom-up method in 2014. These devices behave in a similar way to traditional p-n junctions . Moreove2 and WSe2, are extremely popular 2D materials which were used to build 2D heterostructures. These structures usually have type-II band alignment, which is desirable and can be used to build potential electronic and optoelectronic devices [2–WSe2 and MoS2–MoSe2 LHSs laterally through an epitaxial process with the CVD synthesis method. The built heterostructure has a gradual and seamless interface and exhibits good electronic and optical properties [2–MoS2 heterostructures with a vapor-phase growth process. The monolayer WS2 vertically grown on the MoS2 forms a bilayer heterostructure in high temperature, and the WS2 laterally grown on the edge of MoS2 forms an in-plane heterostructure in a lower temperature [2–MoS2 heterostructure, which is simplified and low cost. The built heterostructure has a high-quality, sharp subnanometer interface. The built-in potential of the heterostructure has been characterized, which is significant in future photoelectrical applications [2/Mox1−WxS2/WS2 LHSs with a concise one-step CVD method on Si substrates have been achieved, by which the formation of different structure components of the LHSs was tuned by the growth temperature and chronology [2  heterostructures with the single-step CVD method [The lateral graphene–h-BN heterostructure was synthesized. In 2010, an h-BN and graphene heterostructure was synthesized using a CVD method, which randomly distributed h-BN and graphene domains in the monolayer hybrid structure. The hybrid structure has a different bandgap compared to those of h-BN and graphene . Fabrica devices . The synoperties . Gong etperature . The latications . The 2D ronology . With thD method . With co2–WS2 heterostructure has been fabricated using the one-step growth technique in ambient pressure [2 is grown first, followed by the epitaxial growth of WS2 doped with Se, and then, the central part is converted into WSe2 because of the overloaded Se. To confirm this, the second and third processes are further observed in an experiment. In the second process, WS2 epitaxial growth is achieved with overloaded S. The Raman image in Fig. 2 in the center region. Se-doped WS2 can be observed at the edge from Fig. 2 in the Raman shift in Fig. 2 is clearly shown in Fig. 2 peak in the Raman spectra in Fig. 2. With this method, graded doped lateral heterostructures can be synthesized through the above-mentioned three sequential processes.Graded doped lateral WSepressure . The oneBecause the mentioned one-step growth techniques are self-assembled, it is difficult to control the shape and size of the heterostructures. In this section, we discuss mainly the two-step growth process based on CVD method, including metal/semiconductor contact  and semiconductor/semiconductor contact (TMDC–TMDC) LHSs, in which most researchers are interested.In 2012, hybrid graphene and hexagonal boron nitride sheets have been synthesized with a two-step CVD method. The graphene was grown from Cu foil, and then, the h-BN grew from the graphene grains on the bare Cu foils . The h-B2 heterostructure synthesized with aromatic molecules as the seed has been reported [2 can grow rapidly along the first layer. The schematic diagram, optical image, and spectroscopy intensity mapping image of graphene–MoS2 heterostructure are given in Fig. 2–graphene heterostructure is fabricated with seed-free direct growth method. The graphene flake is transformed on SiO2/Si substrate, and then, the MoS2 grows along the graphene with atmospheric pressure CVD method. The device based on it has high mobility and low noise, which has better performance than MoS2–metal devices [2 LHS have been fabricated without using seeding. The graphene is synthesized first and then lithographically etched by oxygen plasma. The WS2 is selectively grown on the etched area, forming a film. The LHS has an ohmic contact between graphene and n-doped WS2 [2/graphene LHS has been fabricated with a similar two-step method. The MoS2 was grown between two stripes of graphene on Cu foil. The photodetector based on it has a quick photoresponse and specific detectivity [In 2016, lateral graphene–MoSreported . The pro devices . Large-soped WS2 . A graphectivity .Fig. 8a 2–MoS2, in which both the metal and chalcogen elements are changed at the same time. The two-step growth methods can overcome these difficulties. In 2015, a lateral WSe2–MoS2 heterostructure was synthesized with the two-step lateral epitaxial growth method. The process avoided alloy formation and formed an atomically sharp interface. Although there is a large lattice mismatch, the method provides a reliable means to produce monolayer components for future monolayer electronics [2 was grown first and then the WSe2 epitaxy grew on the edge and on top of the MoSe2 layer. With control over the growth time, different types of heterostructures can be formed. Because the size of each 2D component can be controlled, the two-step methods can grow large heterostructures with sizes of up to 169 µm, and the cross-contamination can be reduced, compared with the one-step growth method [2–WS2 heterostructure with a facile growth process in ambient pressure. The lateral heterojunctions behave as intrinsic p–n diodes and clearly exhibit the photovoltaic effect. The technique can potentially be applied to the growth of TMDC superlattices [With the one-step process, it is difficult to grow p-n heterostructures like WSectronics . Almost h method . Chen etlattices .2–MoS2 and h-BN–MoS2 heterostructures, and the optical image and spectroscopy intensity mapping image of these two heterostructures are depicted in Fig. 2–MoSe2 LHS was synthesized with a two-step CVD method. In this method, the triangle monolayer MoS2 is synthesized first, and then, the MoSe2 is grown along the MoS2 edges, filling the black of the substrate, which forms a clear interface. The photodetector based on the heterojunction film exhibits perfect photoresponse performance. The method can provide a route for other mosaic lateral heterojunction films, which may have optimal photoelectric properties [2 grows first in 730 °C, then the WSe2 grows at 875 °C, the selenium atoms replace the sulfur atoms, and the WSe2 and MoSe2 heterostructure are formed. If the growth is in reverse order, the WSe2 grows at 875 °C first and then the MoS2 grows in 730 °C, and the WSe2 and MoS2 heterostructure can be formed without ion exchange. Using this method, the monolayer MoS2 and WSe2 domain sizes can grow up to 100 µm on SiO2/Si substrates [The method with aromatic molecules as seeds, which can accelerate growth, can also be used to synthesize TMDC–TMDC and h-BN–TDMC lateral heterostructures , as showoperties . A novelbstrates .With the development of the one-step and two-step growth methods, many lateral heterostructures, which grow the second materials at the edge of the first materials, have been synthesized. However, it is still difficult to synthesize lateral heterostructures with multiple distinct materials blocks, which need continuous growth steps . In the 2–WSe2 multi-junction heterostructures have been synthesized by controlling the gas-switching cycles. The number of junctions is determined by the cycles, and the domain size is controlled by the growth time of each cycle. The optical image in Fig. 2–WSe2 heterostructure, which has a size of up to 285 µm. As shown in the inset of Fig. 2 and the brighter region is WSe2. The optical images of five-junction heterostructure with different thicknesses are shown in Fig. 2, and the phone modes of positions 2 and 4 correspond to WSe2. The spatial distribution of the lateral heterojunction can also be confirmed with the Raman spectra. According to the PL spectra, there is a peak at around 1.52 eV for MoSe2 in regions 1 and 3 and a peak at around 1.6 eV for WSe2 in regions 2 and 4. The photoluminescence intensity maps for MoSe2 in 1.52 eV and for WSe2 in 1.6 eV are depicted in Fig. 2 peak at 1.53 eV gradually shifts to the WSe2 peak at 1.6 eV in the first junction (labeled as ‘1’ in the figure). Nevertheless, the junctions in positions 2 and 3 have an abrupt shift, which indicates a sharp interface with fewer alloys. The sharp interface and high-quality crystal are confirmed in Fig. A one-pot synthesis method provides a solution to synthesize lateral multi-junction TMDC heterostructures . MoSe2–W2 junctions with uniform and smooth domain boundaries, which depend mainly on the control of temperature. It is a wonderful example on building layer-controlled large-scale 2D materials and heterojunctions.The CVD method can also be adopted to fabricate thickness-modulated LHSs. Zhang et al. formed bilayer–monolayer (BL–ML) thickness terraces with zigzag orientation lateral heterojunction with the CVD method, with which the second layer TMDC was grown on the monolayer TMDC . The lay2–MoS2 heterojunction has been fabricated with electron beam lithography [2 was first synthesized with CVD method followed by the deposition of SiO2 as a mask, lithographically patterned with e-beam, and had the uncovered part converted into MoS2 by pulsed laser vaporization of sulfur. To totally complete the conversion, the substrate temperature of MoSe2 should be higher than 600 °C, and the laser-vaporized sulfur pulses should be higher than 300. The total conversion is proved by the Raman and PL spectroscopy. The optical and atomic force microscopy images of monolayer MoSe2 are shown in Fig. 2 and converted MoS2 is shown in Fig. 2 completely converted to MoS2 with this method and also lateral heterostructures and array can be formed, which can provide a sharp heterojunction interface (~ 5 nm) with easy spatial control. Different from the conversion method using sulfur plume, the laser-assisted method has been proven to be an effective way to modify the TMDCs, which can replace selenium with sulfur atoms [2 and MoSe2 monolayer, with laser-induced modification in H2S environment. The selenide atoms can change to sulfide atoms in the process with the assistance of a laser whose power is in the range of 0.3–1 mW. The Raman peak intensities of WSe2 in H2S change with exposure time in Fig. 1g mode of WSe2 decreases over time, suggesting the reduction in W–Se chemical bonds; on the contrary, the A1g and 2 increase with time. It is suggested that the sulfur atoms increase in the lattices with exposure time. The Raman and PL spectra for different exposure times in Fig. 2 to WS2 happened with the laser exposure time. This method provides an effective way to construct in-plane lateral heterojunctions.Many other methods have been proposed for the synthesis of LHSs. A lateral MoSehography , as showur atoms , as show2 also have been achieved by introducing local strain [2 is obtained by mechanical exfoliation method from ReSe2 crystal. Then, it is deposited on elastomeric substrates, which are prestretched by 30–100%. The wrinkles on ReSe2 can be formed by releasing the prestrain of elastomeric substrates. The defect tuned by the stress can provide a way to generate electrical gap and tune the optical properties.Strain as an effective way to modify the physical properties of materials is a hopeful technique to realize the LHSs. The strain engineering of materials has been proposed for a long time. The 2D graphene has superior mechanical property, which can sustain elastic deformation of more than 25%. The wrinkles fabricated by applying uniaxial strain lead to additional damping on the graphene. The wrinkles on ReSel strain  in the p2 nanopillars [2 nanopillars in Fig. 2 substrate is etched into nanopillars. The graphene is deposited on the substrate after acid-etching the copper foil under the graphene. Different nanopillar arrays induce different strain distributions in graphene. With the high-density arrays (a < a*), the graphene is suspended between the nanopillars. With the low-density arrays (a > a*), the graphene is fitted on the substrate, with highly symmetric rippers. Depending on the array geometry and pitch, graphene film can conform on the substrate, partially collapse, fakir-like, or suspend. The different configurations of strain domain will provide tunability for the electronic properties.The surface morphology of the substrate can also induce strain on the 2D materials. For example, the strain ripples have been generated by transferring the graphene membrane on the substrate which is corrugated by an array of SiOopillars , as show2 type-I heterojunction [Through CVD, thermal  and plasjunction . This stjunction  built a In this review, we summarized the physical properties of LHSs that can be tuned by the structures , doping, passivation, strain, and dielectric. The device applications and experimental synthesis of LHSs have also been discussed. One-step, two-step, and multi-step growths based on CVD and other growth techniques have been used in the synthesis of heterogeneous junctions. Although there are a large number of works about LHSs, there are still a lot of unsolved issues. For example, in view of the tunable mechanisms, the doping- and passivation-induced heterogeneous junctions are seldom investigated, although the doping and passivation have worked as effective tuning methods in homogeneous junctions. Among the fields of LHSs, researchers pay most attention to the differences in materials in the heterogeneous junctions but neglect the geometrical effects on the properties of heterogeneous junctions. The strain or dielectric modulation has been used to construct homogeneous junctions, but there are very few works published so far. There is a lot of work that needs to be done. In view of the synthesis, the biggest challenge is to develop synthesis technology to fabricate homogeneous junctions where structures, doping, and passivation are precisely controlled. Studies of LHSs offer opportunities to design novel electronic, spin, and optical devices with desirable high performance.


Until now, mixed empirical evidence in experimental and natural studies still leaves many questions about the generality of any single relationship pattern and the underlying mechanisms. Plants are sessile and an enormous portion of their life is located belowground, in particular for herbaceous communities  abundance of saprotrophic fungi is likely to promote plant richness and productivity through increases in mineralization and resource partitioning  is definitely the way forward in understanding how soil‐inhabiting communities affect plant community dynamics and functions. However, typically, roughly half of the sequenced fungal operational taxonomic units (OTUs) can be assigned reliably to species level and for even fewer, function can be assigned. In order to overcome this limitation, efforts to improve databases using cultivation and morphological observation‐based methods, and by adding sequences of known functional groups, are needed.With the results of


N = 138) and fourth (N = 164) graders were tested in the middle (T1) and end (T2) of the school year. The study focused on the cross-sectional and longitudinal development of monitoring and control, and aimed to investigate the development of metacognition for two test formats. After reading expository texts, children completed a comprehension test consisting of open-ended and true-false questions. They monitored their test performance by making confidence judgments, and controlled performance by deciding whether to maintain or withdraw their given answers. Overall, monitoring and control accuracy was higher for open-ended questions than for true-false questions. For open-ended questions, results indicated higher metacognitive accuracy for fourth graders than second graders. No such age effects were found for monitoring and control for true-false questions. Longitudinally, children of both age groups improved their monitoring and control accuracy from T1 to T2, for open-ended and true-false questions. For both test types, improvement mainly occurred for the monitoring and controlling of incorrect, rather than correct answers. Additionally, the results indicated inter-individual stability of performance, but no stability of monitoring and control accuracy over time. The findings indicate that developmental as well as task-related factors affect children’s metacognitive accuracy.This study investigated elementary school children’s development of monitoring and control when learning from texts. Second (The online version of this article (10.1007/s11409-019-09208-5) contains supplementary material, which is available to authorized users. Schools were selected by contacting principals from public schools in the larger vicinity of a mid-sized university town. Family backgrounds were lower to upper middle class. For the analyses, age outliers were excluded (N = 19). Further, sample attrition (N = 6) was due to incomplete test completion, insufficient language skills, eye problems when reading on a tablet, and change in residence. The data of 302 children were used for analyses to test our hypotheses . Sixty-five children (21.5%) were non-native speakers. Outcomes related to our hypotheses did not differ for non-native speakers, indicating that they had sufficient language skills to follow the instructions. Parental informed consent was received prior to testing. Children gave oral consent prior to the test sessions and were informed that they could withdraw participation at any time without consequences. No child ever did.The sample consisted of 327 children at T1 and T2, but also for second and fourth graders. Therefore, we piloted 18 texts with subsets of two open-ended questions and three true-false questions per text. The sample of the pilot study consisted of one second (N = 20) and one fourth grade classroom (N = 15), with children in the same age as children at T2. The texts and questions we used in this pilot study were translated and adapted from studies by De Bruin et al.  of six texts with four questions each, that cover a comparable range of easy, medium and difficult questions. That range in difficulty had to be as similar as possible not only for Tn et al.  and Van n et al. . Texts fThe goal of the pilot study was to select two subsets of six texts with two open-ended and two true-false questions per text that met the following criteria: First, text-question subsets that elicited ceiling or bottom effects in one of the open-ended questions or in more than one of the true-false questions were removed. We expected greater ceiling effects for true-false questions. Therefore, piloting three true-false questions allowed us to exclude one from each text-question set without removing the whole text-questions subset from the sample. Additionally, each sample of true-false questions at both measurement points included 6 true-false questions that were correct and 6 questions that were incorrect. Second, to ensure a suitable range of variance in question difficulty, we chose text-question sets that provided together a similar amount of easy (~ 30%), medium (~ 40%) and difficult (~ 30%) open-ended and true-false questions, for both age groups at both measurement points. And third, to make a comparison between the two age groups most reliable, for both age groups the same subsets of 12 texts (in a difficult and in an easier form) were chosen.The mean length of the 12 selected texts was 87 words for second graders and 126 words for fourth graders. The complexity of the texts as measured by the LIX . Prior to completing the task, children were instructed about the whole task procedure  by one of the experimenters. This instruction took place during the small-groups session, after separating the class in two groups. There were no more additional instructions given during the task, however, children could read instructions for each step and instructions were repeated for individual children if needed. With practical and interactive examples, the experimenters ensured that children understood the task.Each class was visited by four experimenters. At both measurement points the classes were split into two groups of 6 to 11 children, and each group was then tested in an individual room with two experimenters present. Shortly after completing the first measurement point in all classes, teachers were contacted by email to schedule a date for the second measurement point (number of days between the two measurement points: The text comprehension task included the following phases: 1) study (text reading)For the study phase, students read six expository texts on a tablet. Presenting the texts on a tablet enabled a complete randomization of the texts. Further, it standardized the reading procedure for children by not allowing them to move back and forth between texts. The study phase was self-paced; however, the minimum reading duration before having the option to move on to the next text was 10s, ensuring that children could not click through. The computer displays were 11.6 “monitors with a resolution of 1366 × 768 pixels. The text font was Futura Std. Book and the size was 30 for second graders and 25 for fourth graders. Complete texts were presented on screens; no scrolling was needed. The screen for each text contained the title of the text, the text, and a content-related picture. Topics were animals , geographical/ geological/ meteorological subjects , or physiological processes . An example of a text is presented in Appendix 1 and .60 at T2 for second graders, and .68 on T1 and .63 at T2 for fourth graders. Internal consistency for the 12 true-false questions was Cronbach’s α = .37 at T1 and .16 at T2 for second graders, and .45 on T1 and .20 at T2 for fourth graders. Internal consistency of the true-false questions is rather low, however, low internal consistency is not uncommon for text comprehension tests .The text comprehension test consisted of twelve open-ended and twelve true-false questions  presented in a booklet (for examples of questions see Appendix very unsure) to black (very sure), with different shades of grey in the middle  and T2 .To assess performance, answers to true-false and open-ended questions were scored as correct (1) or incorrect (0). In accordance with Van Loon et al. , compreh1 vs. T2) as within-subject factors and age (second vs. fourth grade) as a between-subjects factor were conducted for mean CJs and mean percentages of correct and incorrect answers maintained.To assess monitoring accuracy, mean CJs for correct and incorrect answers were compared to see whether children could discriminate between their correct and incorrect responses  as a within-subject factor and age (second vs. fourth grade) as a between-subjects factor were conducted to investigate whether there were effects of measurement point and age on monitoring and control. Stability between T1 and T2 was assessed with Pearson correlations for performance (percentage of correct answers) and accuracy of monitoring and control (gamma and phi correlations). To investigate the effects of test format, all analyses were conducted separately for both test formats  and true-false questions (M = .19%).Only completed test items were included in the analyses. In line with Roebers et al. , questioIn this section, we report the analyses testing the hypothesized effects of age group, measurement point and test format. Before reporting findings on accuracy and stability of monitoring and control, we report preliminary results on test performance for the two test formats.To investigate differences in performance between age groups and measurement points, we conducted repeated measures ANOVAs for both test formats separately. Table 2 compared to T1, F  = 23.73, p < .001, ηp2 = .07, whereas no difference was found between second and fourth graders (p = .35). The significant interaction between measurement point and age, F  = 12.73, p < .001, ηp2 = .04, showed that second graders’ increased their performance from T1 to T2, F , p < .001, ηp2 = .17, whereas fourth graders’ performance did not change (p = .295). For the number of incorrect answers, no differences were found between T1 and T2, and between second and fourth graders (ps ≥ .32). Children had fewer omission errors at T2 compared to T1, F  = 28.01, p < .001, ηp2 = .09, whereas no difference was found between second and fourth graders (p = .63). The significant interaction between measurement point and age, F  = 13.19, p < .001, ηp2 = .04, revealed that second graders reduced the number of omission errors from T1 to T2,F  = 28.02, p < .001, ηp2 = .17, whereas fourth graders did not show changes in the number of omissions (p = .15).For open-ended questions, children had more correct answers at T2 compared to T1, F  = 73.18, p < .001, ηp2 = .20, whereas no difference was found between second and fourth graders . Performance for true-false questions was above chance for second graders at T1, t (137) = 5.54, p < .001, Cohen’s d = .47, and T2, t (137) = 13.42, p < .001, Cohen’s d = 1.14, and for fourth graders at T1, t (162) = 5.84, p < .001, Cohen’s d = .46, and T2, t (163) = 13.22, p < .001, Cohen’s d = 1.03.For true-false questions, children had more correct answers at Tz = 1.788, p = 0.04) as well as fourth graders .Table 2 compared to T1,F  = 7.91, p = .005, ηp2 = .03, and fourth graders gave lower CJs compared to second graders, F  = 8.03, p = .005, ηp2 = .03. For true-false questions, there were no significant effects of measurement point and age (ps ≥ .47).To investigate effects of age and measurement point on CJs, repeated measures ANOVAs were conducted for open-ended and true-false questions separately. Table 2 compared to T1, we included correctness (correct vs. incorrect) in the repeated measures ANOVAs.To investigate if monitoring was more accurate for fourth graders than second graders, and more accurate at TM = 4.08, SE = .07) than for correct answers , F  = 461.14, p < .001, ηp2 = .64. Consistent with the hypothesis that fourth graders more accurately monitor their incorrect answers than second graders, the interaction between age and correctness was significant, F  = 19.52, p < .001, ηp2 = .07. Fourth graders gave lower CJs for incorrect answers  than second graders , F  = 20.48, p < .001, ηp2 = .07, whereas CJs for correct answers did not differ between second  and fourth graders , p = .32. Further, in line with the hypothesis that children improve their monitoring of incorrect answers from T1 to T2, the interaction between measurement point and correctness was significant, F  = 39.72, p < .001, ηp2 = .13. Children gave lower CJs for incorrect answers at T2  than at T1 , t (285) = 6.62, p < .001, Cohen’s d = 0.78, whereas there was no change over time for CJs for correct answers from T1  to T2 , p = .18. The finding that monitoring of answers to open-ended questions improved for older children and at the second measurement point is also supported by analyses using Gamma correlations, as shown in Table F  = 20.31, p < .001, ηp2 = .08, and children more accurately monitored their performance at T2 compared to T1, F  = 17.50, p < .001, ηp2 = .07.Figure M = 4.96, SE = .07) compared to correct answers , F  = 96.16, p < .001, ηp2 = .25. No significant interaction was found between age and correctness, indicating that second and fourth graders did not differ in their ability to discriminate between correct and incorrect answers to true-false questions, F  = 1.55, p = .22, ηp2 = .01. However, in line with the hypothesis that children improve their monitoring of incorrect answers from T1 to T2, the interaction between measurement point and correctness was significant, F  = 39.57, p < .001, ηp2 = .12. Children gave lower CJs for incorrect answers at T2  than at T1 , t (295) = 3.77, p < .001, Cohen’s d = .44, whereas CJs for correct answers were higher at T2  than at T1 , t (298) = −2.89, p = .004, Cohen’s d = .33. As shown in Table 1 and T2 was also supported with the gamma correlations, F  = 21.94, p < .001, ηp2 = .08.Figure , F 1, 29 = 96.16,1 and T2 with test format as a within-subject factor, collapsed across both grades. The difference between the gamma correlations for monitoring accuracy for open-ended and true-false questions was significant at T1, F  = 44.89, p < .001, ηp2 = .15, and T2, F  = 35.31, p < .001, ηp2 = .11, indicating that monitoring was significantly more accurate for open-ended than for true-false questions at both measurement points.To investigate the differences in monitoring accuracy between open-ended and true-false questions, we conducted repeated measures ANOVAs for Tt (280) = 2.59, p = .01, Cohen’s d = .16. The comparison between CJs for incorrect responses for detail open-ended questions and true-false questions was more pronounced, as indicated by the large effect size, t (289) = −11.94, p < .001, Cohen’s d = .70. Further, a comparison between CJs for incorrect answers to causal open-ended questions and true-false questions also showed a large effect size, t (291) = −14.35, p < .001, Cohen’s d = .88. Thus, even though CJs for incorrect answers were slightly lower for detail than for causal open-ended questions, CJs for incorrect answers to both open-ended question types were considerably lower than CJs for incorrect answers to true-false questions.Note that open-ended questions either had a detail or a causal focus. To investigate whether there are differences in monitoring accuracy for these two types of open-ended questions , we compared monitoring accuracy for these two question types. Moreover, monitoring for both of these open-ended question types was compared with monitoring for true-false questions. As mentioned above, we found differences between question formats for participants` monitoring of incorrect answers. Because this ability to accurately recognize errors is particularly relevant for improving performance, the following analyses focus on differences in CJs for incorrect answers. Table ps ≥ .87). For true-false questions, stability was not significant for second graders (p = .99), whereas for fourth graders, stability over time was low but significant, (p < .05).Table 2 compared to T1,F  = 9.52, p = .002, ηp2 = .03, and overall, fourth graders maintained fewer answers than second graders, F  = 19.65, p < .001, ηp2 = .06. For true-false questions, the effects of measurement point and age were not significant (p ≥ .19).To investigate effects of age and measurement point on the percentage of maintained answers, repeated measures ANOVAs were conducted for both test formats separately. Table 2 than at T1, we included correctness (correct/incorrect answers) in the repeated measures ANOVAs.To investigate the hypothesis that control for incorrect answers would be more accurate for fourth graders than second graders, and the hypothesis that control for incorrect answers would be more accurate at TM = 46.32, SE = 1.5) than correct answers , F  = 331.6, p < .001, ηp2 = .56. Consistent with the hypothesis that fourth graders control their incorrect answers more effectively than second graders, the interaction between age and correctness was significant, F  = 14.24, p < .001, ηp2 = .05. Fourth graders  maintained fewer incorrect answers than second graders , F  = 29.24, p < .001, ηp2 = .10, whereas second graders  maintained more correct answers than fourth graders , F  = 4.46, p = .036, ηp2 = .02. In line with the hypothesis that control of incorrect answers would be more accurate at T2 than T1, the interaction between measurement point and correctness was significant, F  = 20.5, p < .001, ηp2 = .07. Children maintained fewer incorrect answers at T2  than T1 , t (277) = 6.62, p < .001, Cohen’s d = 0.79, whereas the percentage of correct answers maintained did not differ between T1 (M = 73.94 SD = 27.04) and T2 , p = .26. The finding that control for open-ended questions improved for older children and at the second measurement point is also supported by phi correlations. As shown in Table F  = 5.17, p = .02, ηp2 = .02, at both measurement points, and children controlled their performance more accurately at T2 compared to T1, F  = 13.10, p < .001, ηp2 = .06, independent of age.Figure M = 61.99, SE = 1.48) than correct answers , F  = 69.86, p < .001, ηp2 = .19. Again, no significant interaction was found between age and correctness, indicating that second and fourth graders did not differ in their ability to discriminate between control of correct and incorrect answers to true-false questions, F  = .14, p = .71, ηp2 > .001. However, again, and in line with the hypothesis that control of incorrect answers would be more accurate at T2 than T1, the interaction between measurement point and correctness was significant, F  = 17.35, p < .001, ηp2 = .06. Children maintained fewer incorrect answers at T2  than T1 , t (294) = 2.73, p < .001, Cohen’s d = .32, whereas they maintained more correct answers at T2  compared to T1 , t  = −1.97, p = .05, Cohen’s d = −.23. As visible in Table 1 and T2, F  = 5.59, p = .02, ηp2 = .03.For true-false questions, as shown in Fig. , F 1, 29 = 69.86,F  = 12.84, p < .001, ηp2 = .05. Confirming our hypothesis of stronger monitoring-based control in the second measurement point, the results for open-ended questions indicated that the relationship between CJs and control was stronger at T2 compared to T1, F  = 11.73, p = .001, ηp2 = .05. For true-false questions the relation between monitoring and control did not differ between second and fourth graders, and between T1 and T2 (ps > .07).To examine the extent to which children related their control decisions to their CJs (monitoring-based control) we calculated gamma correlations between the CJs and the maintenance of answers  = 22.43, p < .001, ηp2 = .10, and T2, F  = 35.75, p < .001, ηp2 = .13; as visible in Table We also investigated the difference in control accuracy between the two test formats. The difference between control accuracy (Gamma correlations between maintenance and performance) for open-ended and true-false questions was significant at Tps ≥ .90).Table With the current study, we addressed elementary school children’s metacognition when taking a text comprehension test. The first aim was to investigate how monitoring and control develop during the elementary school years by examining developmental effects cross-sectionally and longitudinally. For this purpose, we tested second and fourth grade students twice within half a year with a text comprehension task for which children were asked to read texts, answer questions about the texts, give CJs for their answers, and decide whether they liked to maintain or withdraw each of their answers. The second aim was to examine how test format affects students’ accuracy of monitoring and control. To address this question, the text comprehension test consisted of open-ended and true-false questions.Previous research on young elementary school children’s metacognition mainly used associative learning tasks with forced choice test formats, and individual testing of children. The present study extends this research by using an educationally relevant and more complex text comprehension task. In school, children often need to study texts and answer recall and recognition questions. Several studies investigated elementary school students’ metacognition during reading, and this research shows rather low accuracy of monitoring and control  question difficulty. Thiede  showed tInterestingly, children of both age groups improved their metacognitive accuracy for their answers to true-false questions at the second measurement point. That is, half a year after the first testing, they were less overconfident for, and maintained fewer of their incorrect answers. Thus, improvement over time for true-false questions paralleled the improvement for open-ended questions. The finding that children improved their metacognitive accuracy for true-false questions longitudinally but not cross-sectionally indicates that task familiarity is likely to have an impact on the accuracy of monitoring and control  affects metacognitive accuracy in this task  from familiarity with the specific task by including a control group that completes the task only at the second measurement point.Taken together, our study sheds light on elementary school children’s metacognitive skills when learning from texts. Findings show that the developmental progression of monitoring and control, mainly found in paired associate tasks, also applies to a more educationally valid text-learning task. However, this age-dependent progression was only found when monitoring and controlling open-ended questions. The present study highlights the discrepancy in metacognitive accuracy between open-ended and true-false questions; metacognitive accuracy was considerably higher for open-ended questions. Additionally, it also demonstrated that children’s monitoring and control accuracy varies over time, rather than being a stable characteristic.Investigating the conditions under which metacognition is inaccurate can reveal important insights that can be applied in practice. Findings from intervention studies showed that the promotion of self-regulated learning, which includes metacognition, has an impact on students’ academic performance and metacognitive strategy use


However, atomic level study of PbI2 monolayer has been limited due to challenges in obtaining thin crystals. Here, we use liquid exfoliation to produce monolayer PbI2 nanodisks (30-40 nm in diameter and > 99% monolayer purity) and deposit them onto suspended graphene supports to enable atomic structure study of PbI2. Strong epitaxial alignment of PbI2 monolayers with the underlying graphene lattice occurs, leading to a phase shift from the 1 T to 1 H structure to increase the level of commensuration in the two lattice spacings. The fundamental point vacancy and nanopore structures in PbI2 monolayers are directly imaged, showing rapid vacancy migration and self-healing. These results provide a detailed insight into the atomic structure of monolayer PbI2, and the impact of the strong van der Waals interaction with graphene, which has importance for future applications in optoelectronics.Lead Iodide (PbI Imaging liquid phase exfoliated nanosheets on suspended graphene via annular dark-field STEM can enable identification of various defects, vacancies and their migration. Here, the authors report matching of zigzag edges of monolayer PbI2 with graphene arm-chairs leading to a phase shift from 1 T to 1 H structure to maximize commensuration of the lattices. Although graphene exhibits an exceptionally high carrier mobility (>106 cm2 V−1s−1 at 2 K), its zero bandgap poses difficulties for many semiconductor applications4. Monolayer transition metal dichalcogenides (TMDs), such as MoS2, can have direct band gaps of ~1.8 eV, but do not exhibit very high charge carrier mobility7. For Mo- and W-based TMDs, such as MoSe2, WS2, WSe2, etc., the band gaps fall within the range of 1.0–2.0 eV, which is the red to near infrared regions9. For optoelectronics, blue, green, and ultraviolet light-emitting diodes are also needed for full color displays and cameras. Currently, there are not many experimental studies of 2D materials that can satisfy the demands for the green to UV spectral regions and more research is needed to expand this area.Two-dimensional (2D) materials attract interest because of their unique chemical and physical properties, facilitating the study of novel physics, e.g., trions, valley polarization, etc2 is a layered direct bandgap semiconductor with bandgap of 2.4 eV in its bulk form, whereas its 2D monolayer has an indirect bandgap of ~2.5 eV, with possibilities to tune the bandgap between 1–3 eV13. PbI2 is frequently used to fabricate organic–inorganic halide perovskite solar cells15, and as a high-energy photon detector material for gamma-rays and X-rays18. PbI2 has a wide variety of I-Pb-I stacking and this gives rise to more than a dozen polymorphs19. However, the thickness of each single layer (0.7 nm), and the distance between each lead and iodide atoms (0.32 nm) is independent of the polytypes19. The 2H structure is the most commonly found polytype of three-dimensional PbI2, where each plane of Pb and I atoms are shifted with respect to each other and form overlapping hexagons.PbI2 has been shown to be excellent candidate for use in optoelectronic applications, photodetectors, and photon detection23. Ultrathin PbI2 is an interesting system for studying quantum-confinement effects because the exciton–phonon couplings are dependent on the degree of localization of electronic charge25. Cabana et al.26 fabricated PbI2 interfaces with carbon nanotubes and studied the change in the density of states of the system. Zhou et al.10 studied the graphene/PbI2 van der Waals interface and predicted 1.5 eV increase in the visible light absorption capability of the heterostructure as compared to pure 2D PbI2. Recent work on few layer shows PbI2 as a promising candidate for application in the field of ultrafast saturable absorbers27. However, research into the atomic structure of monolayer PbI2 has been limited to date because of the challenges in obtaining high-quality monolayer crystals and preparing suitable samples for transmission electron microscopy studies. Further work is needed to reveal the structure and dynamics of edges, point defects, vacancy clusters, and nanopores in PbI2 monolayer and its interaction with other 2D crystals, such as graphene.Layered PbI2 monolayer flakes from a starting bulk PbI2 powder. Liquid-phase exfoliation is one of the simplest methodologies for producing 2D materials on a large scale30. In the past decades, various research has been carried out to find suitable solvents based on their interactions with the 2D material to produce suspended 2D monolayer crystals. Surface tension, the Hilderbrand solubility parameter, the Hansen solubility parameter, surface tension components, etc. have been the widely used parameters to screen appropriate solvents35. Using these parameters, we screened the commonly used solvents and found chloroform (CHCl3) to produce monolayer 2D PbI2 crystals. We used annular dark-field scanning transmission electron microscopy (ADF-STEM) to studying the PbI2 atomic structure, by depositing it from solution onto a suspended graphene support and allowing it to dry38. The high electron transparency of graphene, enables provides excellent contrast from the Pb and I atoms in ADF-STEM. We report on the fundamental atomic structure, point vacancies, vacancy clusters, vacancy dynamics, edge terminations and edge etching, and the epitaxial interactions with the underlying graphene support.Here, we used liquid phase exfoliation (LPE) to isolate PbI2 is a unique exception to all the metal halide compounds that show CdI2 structure, in that it has the largest metal halide bond length39. As a result, the bonds are not as ionic as that of the other compounds that also show CdI2 crystal structure, such as MgI2, FeBr2, etc. However, it is sufficiently ionic to dissolve into polar solvents, such as dimethylformamide (DMF), n-methyl-2-pyrrolidone (NMP), dimethyl sulfoxide (DMSO), and to some extent, water41. Recent research on Lewis basicity of solvents, also quantified by Gutmann’s donor number—Dn, has shown that solvents that solubilize the precursor (PbI2) at a total concentration of 1 M, have higher Dn values of >25 (ref. 42). Previous results on the liquid-phase exfoliation for other 2D materials have shown that the most efficient solvents have H-bonding components so as to maximize the dispersability of the material in the solution, which is in agreement with the Hansen solubility parameter theory44. We explored several different solvents and found that chloroform serves as an excellent medium to exfoliate PbI2. CHCl3 has low dipole moment (μ = 1.04), low dielectric constant (ϵT = 20 °C = 4.8), low donor number (Dn = 4), as well as has the ability to form hydrogen bonds47. In addition, CHCl3 has a very low boiling point of 61 °C which facilitates in quick and easy removal after solution dropcasting on to substrates.PbI2. Less sonication leaves multilayer PbI2 dispersed in the solution, whereas longer sonication time renders very small flakes or completely destroys the sheets , where the metal (Pb) atomic plane is in the middle and the two iodide atomic layers at the top and the bottom. A monolayer from a 2 H PbI2 bulk crystal structure  showed the same relative crystal orientation relative to the graphene crystal  results matching the experimental data. To understand why PbI2 adopts 1 H structure with concomitant alignment in these particular directions of graphene, we employed density functional theory (DFT) calculations to study the structural and energetic stability of the different alignments. We used a short-range relaxed atomic model of PbI2 on graphene, as shown in Fig. 2 and leads to these energetically favorable structure that are also observed in the experimental results.Interestingly, we also found that the crystal orientation of PbI2, because edges of 2D materials can influence the electrical, chemical, and catalytic properties due to the different bonding  between atoms54. Fig. 2 flakes, just after being deposited on the graphene. The edges are rough and do not appear to be highly faceted to a specific crystal lattice direction. After electron beam irradiation  transferred by an incident electron (@80 keV) to a single iodide or lead atom is calculated to be 1.38 and 0.85 eV, respectively58. This energy transfer from the incident electron to the specimen atoms is much lower than the required sputtering energy calculated by DFT. Hence, we can deduce that the mechanism of damage in this system is more complicated than purely knock-on damage from atomic sputtering, and likely involves radiolysis. This is not surprising, since PbI2 is a large bandgap semiconductor and therefore prone to ionization effects from the electron beam. Several different types of multiple atom point vacancies are identified, ranging from two atoms  than the single Pb atom (Z = 82), it has lower contrast in ADF-STEM. The line profiles in Fig. 2 structures59.We also observed multiple occasions where the defect Fig.  had one 6f, g,During electron beam irradiation, the vacancies migrated through the lattice and reconstructed. We performed DFT calculations for vacancy migration and found that the energy barrier for migration of lead atom, the in-plane migration of the iodide atom and the out-of-plane migration of iodide atom that corresponded to 0.32, 0.22, and 0.07 eV, respectively , 25% hydrogen in argon (H2), and 100% argon (Ar) were used for graphene synthesis. Before the reaction started, the system was first purged with 1000 sccm Ar, 500 sccm H2, and 100 sccm CH4 for 30 min. The furnace was then heated to 1060 °C with a ramp rate of 50 °C/min accompanied by a flow of 500 sccm Ar and 100 sccm H2. When the temperature reached 1060 °C, the copper was then annealed with the same flow rate for 1 h. Following the annealing process, the synthesis was carried out at 1060 °C for 1 hr with a flow of 500 sccm Ar, 100 sccm H2, and 5 sccm CH4. The furnace was removed from the sample, which allowed the sample to be fast cooled to room temperature.Graphene was synthesized by CVD on a copper substrate. A piece of copper foil was first sonicated in a diluted hydrochloric acid solution (1 mol/L) to remove the oxides on the copper surface. The copper foil was then sonicated in deionized water, acetone, and isopropanol for 5 min each to remove organic residues. To ensure uniformity of temperature during the whole growth process, the copper foil was positioned in the center of a furnace. A total of 1% methane in argon (CH2 powder from Sigma–Aldrich (99% purity) was added to chloroform to form a solution of concentration 1 mg/ml (~0.2 mM). The solution was sonicated in an unheated ultrasonic water bath  for ~30 min to make sure that all the lead iodide had exfoliated and dispersed in the solution. The dispersion was then left to stand for 24 h to let the bulk particles to settle down at the bottom of the container. Then, the supernatant of the dispersion (from the top) was extracted with a pipette and used for preparing TEM samples.PbI2 and vacuum annealing. A few drops (10–20 μl each) were taken from the supernatant of the dispersed solution of 1 mg/ml concentration of PbI2 in CHCl3 and dropcasted on to the TEM grid containing suspended graphene. The TEM holder was then baked in high vacuum at 60 °C to remove all the solvents and other impurities from the surface.Graphene was transferred onto a lacey carbon copper 400 mesh TEM grid using a poly (methyl methacrylate (PMMA) support and then the PMMA removed by acetone. It was then subsequently cleaned by Ar/HRoom temperature ADF-STEM imaging was performed using a JEOL ARM200F located at the David Cockayne Centre for Electron Microscopy (DCCEM) within the Department of Materials at the University of Oxford. An acceleration voltage of 80 kV was used for taking all the images, unless stated otherwise. Imaging conditions used a 30 µm condenser lens (CL) aperture with a convergence semiangle of 22.5 mrad and a beam current of 35 pA. The acquisition angles for these images was 70–220 mrad.2 crystal structure was drawn using Crystal Maker Version 10.3.1.Images were processed using the ImageJ software. Atomic models were constructed using Accelrys Discovery Studio Visualizer. Multislice image simulations were performed using JEMS. PbI63. A 10 × 10 × 1 supercell is used for calculating the sputtering energy and defect migration barriers, and two more different systems are designed to probe the short-range angle dependence of stacking energy: a hexagonal PbI2 flake  on continuous graphene substrate. The systems are varied with different angles between PbI2 and graphene . The elements are represented by projector-augmented wave potentials with 500 eV energy cutoff and the four (6s26p2) valence electrons for Pb, seven (5s25p5) for I, and four (2s22p2) for C are treated explicitly65. Initial relaxation and energetics are calculated via the generalized gradient approximation, Perdew−Burke−Ernzerhof, augmented by DFT-D3 method with Becke–Jonson damping for the interlayer dispersive interaction67. The first Brillouin zone is sampled by the tetrahedron method on a 4 × 4 × 1 k-mesh for the 10 × 10 × 1 supercell, and only gamma point for the angle-dependence studies. All structures are relaxed until the force on each atom is <0.01 meV/Å. The sputtering energy is defined by the energy difference between the crystalline structure and the one with a vacancy, for instance, the I sputtering energy is 69. A total of 10 × 10 supercells and 4 × 4 k-point grids are employed, and a criterion of 0.03 eV/Å is set for the force convergence. For VI migration, two pathways are considered: in-plane and out-of-plane directions, and only in-plane VPb migration is considered. The starting and end points for NEB search are prepared by removing the corresponding atoms from the relaxed pristine structure, and then further relaxed. The six intermediate images are interpolated by the Vasp TST toolkit70. The converged energy paths are the fitted by cubic spline functions.The structural, energetic, and defect migration calculations in this work are based on DFT implemented in the Vienna Ab-initio Simulation Package (VASP)2 intensity contrast, graphene sample without PbI2, different sizes of monolayer PbI2, proof of monolayer flakes instead of bilayer AA′ 1 H structure, T-phase structure on lacey carbon TEM grid, ADF-STEM images of structural transformation immediately after dropcasting on graphene, Moire pattern of 1 T and 1 H atomic models on graphene, zigzag alignment of PbI2 with graphene after edge etching, relative alignment of time frame of PbI2 etching from Fig. Effect of sonication duration and dropcasting, monolayer and bilayer PbISupplementary InformationPeer Review File


Quantifying which assembly processes structure microbiomes can assist prediction, manipulation, and engineering of community outcomes. However, the relative importance of these processes might depend on whether DNA or RNA are used, as they differ in stability. We hypothesized that RNA-inferred community responses to (a)biotic fluctuations are faster than those inferred by DNA; the relative influence of variable selection is stronger in RNA-inferred communities , whereas homogeneous selection largely influences DNA-inferred communities . To test these hypotheses, we characterized soil bacterial communities by sequencing both 16S rRNA amplicons from the extracted DNA and RNA transcripts across distinct stages of soil primary succession and quantified the relative influence of each assembly process using ecological null model analysis. Our results revealed that variations in α-diversity and temporal turnover were higher in RNA- than in DNA-inferred communities across successional stages, albeit there was a similar community composition; in line with our hypotheses, the assembly of RNA-inferred community was more closely associated with environmental variability (variable selection) than using the standard DNA-based approach, which was largely influenced by homogeneous selection. This study illustrates the need for benchmarking approaches to properly elucidate how community assembly processes structure microbial communities. Research on ecological succession is key to advance our understanding of how communities are assembled and affect ecosystem functioning and dynamics. In accordance with classical studies on plant community succession, the relatively recent advances in high-throughput sequencing technologies and microbiome profiling have revealed that microbial communities across a broad range of habitats also undergo sequential changes through different time scales ,2,3.Most studies have profiled bacterial community composition across succession gradients by sequencing 16S rRNA gene from environmental DNA ,4,5,6. HDisentangling the relative influences of community assembly processes structuring the microbiome distribution is important for understanding community turnover during succession . For insQuantifying the assembly processes is often dependent on either taxonomic or phylogenetic information of the investigated meta-community. Interestingly, it was shown that the phylogenetic structure of RNA-inferred bacterial communities is significantly more clustered than that of the DNA-inferred communities, thus resulting in a stronger environmental filtering signal . SelectiIn this study, we investigated the dynamics of bacterial communities across five successional stages in a primary succession soil chronosequence. Soil samples were collected across the successional stages and within successional stages at four time points. The analysis of DNA-derived 16S rRNA gene sequences encompassed the total fraction of bacterial communities including DNA from non-viable cells; and the RNA-derived 16S rRNA sequences encompassed the content of bacterial ribosomes indicating the potentially active fraction of total bacteria communities. We first examined to what extent these two approaches affect the results of bacterial community turnover during primary succession, and then compared the interplay of community assembly processes, and evaluated whether these two approaches affect the outcomes and conclusions of microbial community successional dynamics in soils.The study was conducted in a salt marsh ecosystem at the island of Schiermonnikoog, the Netherlands . This island displays a progressive growth expansion eastwards caused by the continuous sedimentation of particles carried by wind, currents and flooding cycles. This natural chronosequence spans over 100 years of succession, in which early successional stages are located in the east and late stages in the west. Soil physicochemical properties, aboveground biomass, and ecosystem disturbances gradually change from early to late successional stages ,21. For To characterize the spatiotemporal variation of bacterial communities in this system, triplicate soil samples were collected from five successional stages  in May, July, September and November 2017 . At each replicate, 20 soil cores  were randomly sampled. A total of 2 g of the collected soil that was homogenously mixed was preserved in LifeGuard Soil Preservation Solution  and stored at −80 °C for further nucleic acid extraction.The total RNA and DNA were co-extracted from 2 g of soil for each of the 60 samples using the RNeasy PowerSoil Total RNA kit  with the RNeasy PowerSoil DNA Elution kit , following the manufacturer’s instructions . After eWe profiled the DNA- and RNA-inferred bacterial communities by sequencing the V4 region of the bacterial 16S rRNA gene and 16S rRNA transcripts, respectively, using primer pair 515F (5′-GTGCCAGCMGCCGCGGTAA-3′) and 806R (5′-GGACTACHVGGGTWTCTAAT-3′) ,23. Eachde novo phylogenetic tree was generated from representative sequences by aligning sequence fragments via MAFFT, masking ambiguous alignments and inferring a tree using the FastTree algorithm [We used the QIIME2 pipeline (version 2019.10) to process 16S rRNA gene and rRNA sequences . The demlgorithm . A rooteAll subsequent analyses were carried out in R (v3.5.0) ,29. To cbray). We did this by comparing the ASV taxonomic turnover between a pair of communities and the null distribution [bray metric, the Bray–Curtis dissimilarity between observed communities was first calculated. Then, the null distribution of the Bray–Curtis dissimilarity between simulated communities were constructed by randomly sampling ASVs 999 times. The RCbray matrix was generated by comparing the Bray–Curtis dissimilarity between a pair of communities and the null distribution of Bray–Curtis dissimilarity. When |βNTI| < 2 and RCbray > 0.95, community turnover is dominated by dispersal limitation, as the dissimilarity between observed communities is higher than the expectation. |βNTI| < 2 and RCbray < -0.95 indicates homogenizing dispersal, as the community turnover between observed communities is lower than the null expectation. If |βNTI| < 2 and |RCbray| < 0.95, both phylogenetic and taxonomic community turnover of observed communities are not different from the null distributions. In other words, neither selection nor dispersal dominate the assembly processes, their influences on community turnover act together with drift, thus being termed as ‘undominated processes’. Together, we quantify the relative influence of each assembly process by the proportion of each assembly process within each dataset or treatment.We applied a framework that assesses the phylogenetic and taxonomic turnover of communities, and further used null modelling distributions to quantify the relative influences of distinct assembly processes mediating community turnover ,32. In tribution . To creaThe analysis of βNTI relies on the correlation between relatedness of species phylogeny and their ecological niches. We estimated the optimal niche of ASVs (occurrence > 5) with soil pH, soil sodium concentration, soil organic carbon and soil water content using the function ‘wascores’ in the package vegan. Phylogenetic distance across ASVs was generated using the ‘cophenetic’ function in the package picante. Last, we tested the phylogenetic signal  using the function ‘mantel.correlog’ in the package vegan. For all the four soil parameters, significant positive correlations were observed at short phylogenetic distances, confirming the assumption for suitable ecological inferences using short phylogenetic distance based on βNTI .https://github.com/Jia-Xiu/Jia_et_al_Microorganisms_2020.Figures were made using the ggplot2 package . All scrAfter denoising, filtering the low quality, short length and chimeric sequences, as well as removing singleton and non-target taxa, the total dataset consisted of 12,741,738 reads . After rarefying the feature table to 15,000 reads per sample, the rarefaction curves for most DNA-based samples reached a steady plateau, but some RNA-based samples did not, which indicates the sampling efforts were enough for most DNA-based samples, while more sequencing depth was potentially needed for some RNA-based samples . In the R2 = 0.48 and R2 = 0.41, respectively; p < 0.001), followed by sequencing approach  in both DNA and RNA datasets to be Proteobacteria, Bacteroidetes, Actinobacteria, Acidobacteria and Planctomycetes . As ecolbray metric. The results show that selection  dominated the assembly processes of both DNA- and RNA-inferred communities  bacterial cells contain more ribosomes than inactive cells . In thisThe quantification of community assembly processes using both DNA- and RNA-based approaches was dominated by selection. In relative terms, the influence of variable selection was higher in the RNA-inferred communities, suggesting that RNA-inferred communities more rapidly respond to environmental fluctuations compared to a more ‘stable’ scenario of DNA-based communities. This is consistent with a previous finding showing that the phylogenetic community structure of RNA-based communities changes quickly in response to variations in pH and carbon to nitrogen ratios . MoreoveThe broad use of amplicon sequencing has greatly advanced our understanding of the ecological processes structuring bacterial communities during succession. However, DNA- and RNA-based approaches can generate distinct profiles of community composition. This can be caused by differences in the stability of DNA and RNA, differences in the copy numbers of the 16S rRNA gene, in addition to changes in the number of transcripts of this gene, i.e., differences in cell active states and lifestyle. Here, we used these two approaches to profile bacterial communities across a primary successional gradient and quantified the relative influences of community assembly processes governing community turnover. Our results demonstrate that RNA-based communities have greater variation in community composition and are relatively more influenced by variable selection; while DNA-inferred communities have less variation and are relatively more influenced by homogeneous selection. Future studies advancing knowledge on the community assembly of bacterial communities and successional dynamics must be cautious when interpreting data obtained from either DNA- or RNA-based sequencing approaches.


Gas hydrate technology holds great potential in energy and environmental fields, and achieving efficient gas hydrate formation is critical for its industrial application. Graphene is a novel carbon-based nanostructured material with excellent thermal conductivity and a large specific surface area. Therefore, the use of graphene-based materials for the promotion of gas hydrate formation might be feasible and has aroused a lot of interests. Accordingly, to evaluate the current research on graphene-based promotion of gas hydrate formation, this work presents a review of existing studies involving graphene-based promoters of gas hydrate formation. Here, the studies applying various types of graphene-based promoters for gas hydrate formation are listed and detailed, the peculiar properties of graphene-based promoters are discussed, and the promotion mechanisms are analyzed. Through this review, comprehensive insight into graphene-based promotion of gas hydrate formation can be obtained, which can guide the design and applications of novel graphene-based promoters and might contribute to achieving efficient gas hydrate formation. Gas hydrates have captured an increasing amount of attention during the past decades because of their great potential for energy storage and environmental conservation  reduced the hydrate formation period by 45–80% and improved the hydrate formation rate and the storage capacity by 190–660% and 45–70%, respectively, compared with pure water. The results implied that the graphene nanosheets not only increased heterogeneous nucleation in the system and provided abundant active sites for hydrate nucleation but also produced a high transfer efficiency that could remove the heat generated by hydrate formation from the system, consequently improving the efficiency of methane hydrate formation. Due to similar promotion mechanisms, graphite nanoparticles also have positive effects on gas hydrate formation. Zhou et al.  compared in a pure water system. Adding SDBS into GN nanofluid could inhibit GN aggregation and greatly reduce the surface tension of the solution, making gas molecules dissolve in water more easily, which favored CO2 hydrate formation.In addition to the direct promotion of gas hydrate formation, graphene can also be mixed with surfactants to generate surfactant-stabilized nanofluids. Graphene usually exhibits poor dispersity and stability during gas hydrate formation, causing weak performance and cyclability during promotion. Therefore, a mixture of graphene and surfactants has been employed, where the surfactants function as both stabilizer and co-promoter. Hosseini et al.  used SDS−1 of SGO, and the storage capacity reached 140–150 v/v  onto surfaces of graphene/GO, aiming to obtain novel graphene-based promoters with desirable properties to significantly promote gas hydrate formation. Moreover, because of its nanostructure and remarkable electrical conductivity, graphene might serve as a nano-sized electric rotor under an electric field that could effectively stir within nano-sized confinement spaces, which might possibly be applied for HFP promotion via improving mass transfer.The existing studies on graphene-based promotion of gas hydrate formation were implemented in lab-scale experiments, so the promotion effects, stability, and cyclability of graphene-based promoters in gas hydrate formation need to be investigated in pilot tests, which could be conducted in future work. Additionally, further research can focus on grafting graphene/GO with functional groups to produce exceptional CMG or introducing functional nanoparticles (e.g., Ag and FeM-TS wrote and revised the manuscript. FW and G-DZ supervised and revised the manuscript. All authors contributed to the article and approved the submitted version.The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.


The multiple micronutrient supplement (MMS) based on the United Nations International Multiple Micronutrient Antenatal Preparation (UNIMMAP) formula provides women and their offspring with a healthy start to life in an efficacious, safe, and cost‐effective way. To date, however, no precise and transparent specifications exist to support the manufacturing and distribution of UNIMMAP–MMS globally. To palliate for this need, the MMS Technical Advisory Group at the New York Academy of Sciences and the Micronutrient Forum convened a technical consultation to develop an open access UNIMMAP–MMS Product Specification for the manufacturing of this product. The specifications offered in this paper cover: ingredients, excipients, and processing aids used in the manufacturing of UNIMMAP–MMS; stability studies recommended under different testing conditions and climatic zones; packaging considerations; manufacturing standards, including pharmacopeia standards, manufacturing practices, certificates of analysis, change control, and quality agreement; finished product specifications, including tablet characterization and purity, potency assay; analytical test methods; and storage and transportation requirements. This paper results from deliberations of a panel of international experts assembled in Washington DC, November 11 and 12, 2019 and commissioned by the MMS Technical Advisory Group at the New York Academy of Sciences and the Micronutrient Forum to ensure consistency in the manufacturing of quality and affordable UNIMMAP–MMS across antenatal care programs, while allowing for variances in national regulatory and pharmacopoeia standards. Micronutrient deficiencies can have deleterious impacts on maternal health and pregnancy outcomes.1aThis specification states a preference for a film coated tablet, which provides certain benefits, including lower cost and better performance and stability under expected conditions of high temperature and humidity. However, capsules that demonstrate equal or better performance may also be considered.The product defined by the following specification conforms to the UNIMMAP) formula and is an MMS for pregnant women that is delivered in the form of a film coated tablet.22.1bReaders should consider the following: (1) UNIMMAP–MMS might be considered a medicinal product in some countries, and if so, the product must comply with the respective regulatory requirements of that country; (2) no reference is made in this document to “pharmaceutical” or “medicinal” ingredients or products, although the product might be considered a medicinal product in some countries; and (3) this specification outlines the minimum requirements for the manufacture of a UNIMMAP–MMS; as previously indicated, if a country has stricter requirements, they must be met. Where such standards do not exist, ingredients may be used in the UNIMMAP formulation if they have been shown to be of acceptable food grade quality using other suitable procedures.Table 2.2Excipients used in the UNIMMAP formulation generally are prepared from ingredients that meet USP, National Formulary (NF), Food Chemical Codex, or other globally recognized pharmacopeia compendial standards. Where such standards do not exist, ingredients may be used in the UNIMMAP formulation if they have been shown to be of acceptable food grade quality using other suitable procedures.Ingredients may be added to the UNIMMAP formulation provided that the ingredients comply with applicable regulatory requirements, and do not interfere with the assay and tests prescribed for determining compliance with the bulk or finished UNIMMAP–MMS product specification.2.3Processing aids or other materials used in the manufacture of the UNIMMAP formulation that do not end up in the finished product should be of acceptable food grade quality using suitable procedures.purified water.Potable water must meet, at minimum, all the requirements for drinking water promulgated in the U.S. Environmental Protection Agency's National Primary Drinking Water Regulations (40 CFR Part 141), and any applicable state and local drinking water requirements that are more stringent. For manufacturers outside the United States, potable water meeting equivalent requirements may be acceptable with justification, for example, the drinking water regulations of the European Union (European Commission Directive 98/93/EC) or Japan Drinking Water Quality Standards. Water not meeting such requirements should not be permitted for use in the water purification system for 3The UNIMMAP–MMS finished product labeling must state a shelf‐life (expiration) date that is indicative of the date before which the product is ensured to meet applicable specifications of identity, strength, quality, and purity when stored under labeled conditions. The shelf‐life (expiration) date must be supported by suitable stability data, following the guidelines in International Council for Harmonisation of Technical Requirements for Pharmaceuticals for Human Use (ICH) Q1A.A documented ongoing testing program must be designed to monitor the stability characteristics of the UNIMMAP–MMS, and the results must be used to establish appropriate storage conditions and shelf‐life (expiration) dates for the UNIMMAP–MMS finished product. Test procedures used in stability testing must be validated and indicate stability. Stability samples should be stored in container‐closure systems that simulate the packaging proposed to distribute the finished UNIMMAP–MMS for consumer/patient use. Stability studies should include testing of those attributes of the dietary supplement that are susceptible to change during storage and that influence the quality of the dietary supplement.The first three production batch(es) should be placed on the stability monitoring program to establish the product shelf‐life (expiration) date. The lots should be those that are manufactured at the regular manufacturing scale; however, two of the three production batches can be at least 1/10th the size of the manufacturing scale. Thereafter, at least one batch per year of manufactured UNIMMAP–MMS should be added to the stability monitoring program. All batches must comply with the finished product specification throughout the product shelf‐life (expiration) date.As appropriate, the stability storage conditions for temperature and relative humidity for product intended for use globally should be for Climate Zone IVb, hot and very humid. However, actual climatic conditions in the country of destination of the UNIMMAP–MMS may require stability studies to be carried out under the conditions of a different climatic zone . Table Long term = 0, 3, 6, 9, 12, 18, 24, 30, and 36 months;Accelerated = 0, 3, and 6 months.The frequency of testing should be sufficient to establish the stability profile of the dietary supplement. The storage conditions and length of studies chosen should be sufficient to cover storage, shipment, and subsequent use. Data from the accelerated storage condition can be used to evaluate the effect of short‐term excursions outside the label storage conditions, such as might occur during shipping. The shelf‐life (expiration) period of the UNIMMAP–MMS should be 30 months, at minimum. The following testing frequencies are recommended:Where an expectation (based on development experience) exists that results from accelerated studies are likely to approach significant change criteria, increased testing should be conducted by adding samples at the 1‐ and 2‐month time points. In general, a significant change for a dietary supplement is defined as a 5% change in the assay from its initial value; or failure to meet any of its product specification acceptance criteria.44.1cNo evidence currently exists yet in the literature indicating that packaging type/tablet count increases uptake, adherence rates, or clinical attendance. However, the 180‐count bottled product makes it less expensive per tablet to dispense and has a lesser environmental impact than other packaging. For these reasons, the 180‐count‐HDPE‐bottled product was selected as the packaging option of choice in the preparation of this document. If future implementation research, or if program needs indicate otherwise, other packaging options may be considered with appropriate modifications to these technical specifications.This specification focuses on the use of UNIMMAP–MMS packaged in bottles containing 180 tablets per bottle. Bottles (or blister packaging) containing 30 tablets are acceptable, but packaging options/tablet counts should be considered in light of cost and environmental implications, and evidence that may come later as to the impact that a particular packaging option or tablet count may have on distribution, uptake, adherence rates, or clinic attendance.Bulk packaging for business‐to‐business transactions is acceptable with demonstrated stability. Bulk packaging for clinics  should be avoided due to health and safety concerns to avoid inadvertent child exposure to UNIMMAP–MMS; and to avoid repackaging by clinic staff of MMS into other temporary less desirable packaging , which may cause premature product deterioration.4.2White/opaqueScrew capHigh‐density polypropylene (HDPE) material Tamper evidentChild resistantBottles must be:The need for desiccant depends on tablet formulation and must be determined by the manufacturer based on experience and supporting stability data.4.3®, should be used to ensure the stability of the product throughout the shelf‐life of the product. Using a child‐resistant version of a blister pack is deemed impractical, as the “child‐resistant” requirement makes a blister pack unusually difficult to dispense package contents.If blister packaging is used, a thermoformable moisture barrier film, such as Aclar4.4Labeling must comply with applicable country of destination regulatory requirements for the food/dietary/nutritional supplement.Quantitative label claims for the product must be truthful and accurately reflect the contents of the declared food/dietary/nutritional ingredients; fortified or fabricated nutrients must meet 100% of the quantitative label claim throughout the shelf‐life of the product for products that can be distributed in the United States. For products distributed outside the United States, fortified or fabricated nutrients must meet USP compendial assay acceptance criteria . The label must include a domestic address or phone number through which an adverse event report for a dietary supplement may be received.55.1United States Pharmacopoeia (USP);European Pharmacopoeia (Ph.Eur.);International Pharmacopoeia (Ph.Int.);British Pharmacopoeia (BP);Japanese Pharmacopoeia (JP).Compliance to the following international pharmacopoeial standards is acceptable for food/dietary/nutritional supplement ingredients and finished product:5.2U.S. FDA 21 CFR Part 111 Current Good Manufacturing Practice in Manufacturing, Packing, Labeling, or Holding Operations for Dietary Supplements; andUSP–NF general chapter 〈2750〉 Manufacturing Practices for Dietary Supplements.The UNIMMAP–MMS must be manufactured under current good manufacturing practice (cGMP) regulations promulgated by an internationally recognized regulatory authority , or other GMP guidelines by a stringent authority ), by a PIC/S member, or by a globally recognized pharmacopeia , including but not limited to:The manufacturing site and operations should be audited by an accredited third‐party certification body.5.3Certificates of analysis (CoA) must be issued for each batch of UNIMMAP–MMS product. The CoA should list the name and item code of the UNIMMAP–MMS product, batch number, tested and released dates, and the expiration date. The CoA should list each test performed, the specific identity of the test procedure, the acceptance limits or criteria, and the results with numerical units, as appropriate. The CoA should be dated and signed by authorized quality unit personnel. For CoAs issued for external use, the CoA should list the name, address, and telephone number of the manufacturer.The testing protocol should include the performance of full specification testing. However, a reduced level of testing (or sampling) for particular specified parameters may be allowed based upon one or more of the following: statistical analysis of an adequate quantity of historical test data; statistical confidence in the capability of the manufacturing process as determined by suitable verification; or ongoing monitoring of the process using recognized statistical process control techniques. The manufacturer must notify the purchaser of the UNIMMAP–MMS product in writing of any change to the testing protocol.5.4The UNIMMAP–MMS product may be manufactured meeting Halal requirements. The exact requirements shall be obtained from local authorities or from an accredited source.5.5A manufacturer might make changes to the UNIMMAP–MMS product's specification, raw material source, manufacturing processing steps and/or equipment, testing protocols, or any other criteria deemed to be essential or significant to product quality. Manufacturers must notify the purchaser of the UNIMMAP–MMS product of any significant changes that might affect product quality, in writing upon implementation, along with the rationale for the change(s).A manufacturer must also notify the purchaser of the UNIMMAP–MMS product of any change to its manufacturing site, including any changes to the certification status held by the manufacturer from a GMP issuing authority.5.6There should be a written and approved contract or quality agreement between the contract giver and the contract acceptor that defines in detail the GMP responsibilities, including the quality measures, of each party. The contract should permit the purchaser to audit the manufacturer's (seller's) facilities for compliance with GMPs. Where subcontracting is allowed, the seller should not pass to a third party any of the work entrusted to it under the contract without the buyer's prior evaluation and approval of the arrangements.6The UNIMMAP is a formulation for use by pregnant women. UNIMMAP–MMS was developed during a workshop of experts organized by the WHO, UNICEF, and the United Nations University in 1999 specifically to identify an MMS formula for efficacy clinical trials. It contains 15 micronutrients at dosages that approximate the recommended dietary allowances for pregnancy.Tables 7Tests and examinations that are used to determine whether UNIMMAP–MMS specifications are met must be appropriate for their intended use, and scientifically validated methods. Test methods or procedures must meet proper standards of accuracy and reliability.If the test procedure is not in an official compendium, the procedure must be validated according to USP general chapter <1225> Validation of Compendial Procedures or ICH Q2(R1) Validation of Analytical Procedures: Text and Methodology. Method performance characteristics include specificity, linearity, range accuracy, precision, detection limit, and quantitation limit, and those of interest may vary depending on the type of test: identification, assay, impurities, or performance.If the test procedure is in an official compendium, such as USP‐NF, the procedure only needs to be verified for its suitability under actual conditions of use, according to USP general chapter <1226> Verification of Compendial Procedures. Verification requirements should be based on an assessment of the complexity of both the procedure and material to which the procedure is applied. Verification is not required for basic compendial procedures, such as loss on drying, residue on ignition, and simple instrumental determinations, such as pH measurements.An alternative method or procedure is defined as any method or procedure other than the compendial method or procedure for the article in question. The alternative method or procedure must be fully validated and must produce comparable results to the compendial method or procedure within allowable limits established on a case‐by‐case basis. Alternative methods or procedures can be developed for any number of reasons not limited to simplification of sample preparation, enhanced precision and accuracy, improved (shortened) run time, or being better suited to automation than the compendial method or procedure. Only those results obtained by the methods and procedures given in the compendia are conclusive.8The UNIMMAP–MMS product must be held under appropriate conditions of temperature, humidity, and light so that its identity, purity, strength, and composition are not affected .The UNIMMAP–MMS product must be distributed under conditions that will protect it against contamination and deterioration. The manufacturer and purchaser need to work together to ensure that this requirement is met.Segregation, isolation, or the use of packaging to protect the UNNIMAP‐MMS product from contamination from other articles in the same load;Use of vehicles and transportation equipment that are adequately designed and maintained in a sanitary condition to prevent the UNIMMAP–MMS product from becoming contaminated during transportation operations; andUse of vehicles and transportation equipment that are adequately designed, maintained, and equipped to transport UNNIMAP‐MMS product under adequate temperature and humidity control to prevent UNNIMAP‐MMS product from becoming adulterated during transportation.All transportation operation must be conducted under such conditions and controls necessary to prevent UNIMMAP–MMS product from becoming adulterated during transportation. Responsibility for ensuring that transportation operations are carried out adequately must be assigned to competent supervisory personnel. Shippers, receivers, loaders, and carriers engaged in transportation must conduct all transportation operations under such conditions and controls necessary to protect the UNNIMAP‐MMS product from becoming adulterated during transportation. Such operations include, but are not limited to, taking effective measures such as:The first manufactured batch of the UNIMMAP–MMS product should be distributed first. Distributing operations must be designed to facilitate its recall, if necessary.9The authors declare no competing interests.