Over the past decades, the design of active catalysts has been the subject of intense research efforts. However, there has been significantly less deliberate emphasis on rationally designing a catalyst system with a prolonged stability. A major obstacle comes from the ambiguity behind how catalyst degrades. Several degradation mechanisms are proposed in literature,   but with a lack of systematic studies, the causal relations between degradation and those proposed mechanisms remain ambiguous. Here, a systematic study of a catalyst system comprising of small particles and single atoms of Pt sandwiched between graphene layers, GR/Pt/GR, is studied to  unravel the degradation mechanism of the studied electrocatalyst for oxygen reduction reaction(ORR). Catalyst suffers from atomic dissolution under ORR harsh acidic and oxidizing operation voltages. Single atoms trapped in point defects within the top graphene layer on their way hopping through toward the surface of GR/Pt/GR architecture. Trapping mechanism renders individual Pt atoms as single atom catalyst sites catalyzing ORR for thousands of cycles before washed away in the electrolyte. The GR/Pt/GR catalysts also compare favorably to state‐of‐the‐art commercial Pt/C catalysts and demonstrates a rational design of a hybrid nanoarchitecture with a prolonged stability for thousands of operation cycles. Catalyst protection through sandwiching between two graphene sheets enhances stability without compromising the activity to catalyze oxygen reduction reaction in acidic medium. Spectroscopic and computational analyses reveal the catalyst degradation mechanism. Under oxygen reduction reaction (ORR) conditions, evolution of point defects in graphene top layers provides a tortuous path for single atoms to reach the catalyst surface to catalyze ORR. Another strategy is forming a core@shell architecture where Pt is the shell layer and secondary metals act as a core support. Through crystal mismatch, mechanical strain induced on Pt shell enhances its activity. Nevertheless, these attempts fail in transcending the activity‐stability dilemma: the higher activity achieved often degrades the stability. Transition metals  are less stable under harsh acidic environment and highly oxidizing potential, under which PEMFC operates. Different degradation mechanisms have been proposed in literature, such as catalyst atoms dissolution, particle detachment from the support, Ostwald ripening, particles agglomeration, or catalyst‐support corrosion. Those mechanisms could happen simultaneously with different kinetics according to the catalyst design.Proton exchange membrane fuel cell (PEMFC) has the oxygen reduction reaction (ORR) as the key electrochemical reaction at the cathode side. Three objectives when developing new electrocatalysts either for ORR or other electrochemical reactions must be well optimized: enhancing the catalytic activity, reducing the cost of the catalyst, and increasing its lifetime. In general, over three decades scientists have been paying significant attention to design alternative electrocatalysts with higher activity and reduced cost. Numerous efforts have been devoted, when designing a new ORR electrocatalyst, to increasing its activity through strain engineering, and reducing the cost by replacing the expensive Pt component with other earth abundant transition metals. A prominent strategy is alloying of Pt with other secondary metals . The effect of graphene cap thickness (the number of layers) on catalyst stability and electrocatalytic activity for ORR has been systemically studied. A combination of experimental and computational analysis presented to unravel the degradation mechanism of electrochemical active surface area (ECSA) and identify the origins of electrocatalytic activity enhancement. Capping metal catalyst with three‐layer graphene thick sheet showed optimal performance with transcending catalyst stability over 20K durability testing without sacrificing ORR activity. The synthesized architecture is flexible and can be extended to catalyze a wide variety of electrochemical reactions, where stability is maximized, without compromising the catalytic activity.2FigureTableThree sample sets synthesized with Pt small nanoparticles sandwiched between two sets of graphene sheets (GR/Pt/GR), as shown in 2.1Figure The interplanar distance deduced from FFT  with an accelerating voltage 300 and 200 KeV, respectively, to interrogate the morphology of Pt structures and their dispersion on 2D graphene films. Sputtering power and deposition time were varied incrementally to monitor the growth mode of Pt nanoparticles on graphene. Detailed experimental setup and synthesis optimization can be found in the Supporting Information. A set of samples was sputtered for 6, 15, 26, and 60 s at 3 W power. s Figure  were obsm HClO4 with a negative going sweep (0.9: −0.1 V vs Ag/AgCl) at 10 mV s−1 scan rate. Different graphene top layer  was used  to demonstrate the effect of graphene thickness coating on ORR activity, and in the following section we are establishing the same comparison in regard of catalyst stability. Samples with five‐layer thick graphene  showed no ORR activity. On the other hand, samples with single layer graphene  showed clear activity toward ORR, which agrees with other literature reports confirming ORR activity for similar case scenario. Surprisingly, GR3/Pt/GR sandwich structure with three‐layer thick graphene showed noticeable catalytic aptitude toward ORR. Mass activity of sandwich structure outperformed that of state‐of‐the‐art commercial Pt/carbon catalyst counterpart. As a universal metric to evaluate ORR mass activity, current density was reported at 0.9 V versus RHE reference electrode. GR3/Pt/GR sandwich structure showed twofold enhancement in mass activity and 35 mV less overpotential at half‐wave of LSV, compared to state‐of‐the‐art commercial Pt/carbon catalyst, as shown in Figure Two desired objectives for synthesized commercially competitive catalyst are prolonged stability and increased activity. Catalytic activity of GR/Pt/GR sandwich structure was evaluated toward ORR. ORR catalytic activity was tested in an oxygen saturated 0.1 scenario.17 Sur FFT and image analysis of TEM images showed contraction of Pt─Pt bond distance with ≈4%, observed earlier for Pt grown on graphene when compressive strain was induced on Pt adatoms. This was reflected with an increase in GR3/Pt/GR intrinsic activity compared to state‐of‐the‐art Pt/C catalyst. Pt metal is at the strong Pt‐oxygen interaction side of the volcano plot. To enhance ORR activity, oxygen bonding to Pt should be weakened to facilitate the desorption of ORR reaction intermediates. Compressive strain affects metal adatoms d‐band center position, pushing d‐band center down (further away) from the Fermi level. Thus, antibonding states of metal adatoms are more filled where interaction with reaction intermediates  becomes weaker. In addition, the synthesized mixed single atomically dispersed and small nanoparticles promoted higher precious metal catalyst  utilization with larger surface area exposed per mass loading of Pt. However, stability of nonchemically bonded single Pt atoms was shown to be significantly weaker compared to uniform wetted Pt layers on graphene .GR3/Pt/GR activity enhancement is believed to originate from the intimate contact between Pt and graphene, where Pt─Pt is affected by templated growth on graphene.19 FFT2.2upd) peaks’ area (0.0: −0.2 V vs Ag/AgCl as a ref electrode). Electrochemical testing was performed in a nitrogen saturated 0.1 m HClO4. Cyclic voltammetry (CV) characterization sweeps were obtained at a rate of 50 mV s−1 from −0.2: 1.0 V versus Ag/AgCl. Direct comparison between state‐of‐the‐art commercial Pt/carbon (HISPEC4000) and GR/Pt/GR sandwich structures showed superior catalyst stability for our sandwich structure for all graphene‐capped samples regardless of their thickness. Accelerated durability testing (ADT) results showed good stability for GR1/Pt/GR sandwich structure, surviving ≈78% of ECSA after 5K cycles; however, ECSA suffered from rapid ECSA loss in the subsequent cycles up to 15K cycles. This is plausible due to presence of defects within graphene, as will be discussed in detail with an aid of an ex situ Raman analysis.ECSA was tracked through observing changes of hydrogen waves (Hectrode).35 Eleupd (−0.2: 0.0 V vs Ag/AgCl) was observed, similarly no observation of Pt─O reduction peak (0.4: 0.8 V) were cited, however, X‐ray photoelectron spectroscopy (XPS) analysis confirmed Pt presence within the surface vicinity. Increasing ADT cycles showed pronounced Pt─O reduction peak around 0.65 V, with monotonic signal enhancement during ADT cycling up to 30K cycles. Simultaneously, no Hupd was observed during ADT cycling up to 30K cycles. Meanwhile, pyrolytic graphene defect's peak was observed during positive CV sweeps at 0.9 V, indicating continuous increase of defect's density during ADT.ADT results showed tremendous stability of GR3/Pt/GR sandwich structure with ECSA increasing by 5% and 7% during first 1K and 5K testing cycles, respectively. Sandwich structure with three‐layer graphene survived 84% and 75% of original ECSA after 15 and 20K ADT cycles. This result is a key feature toward establishing state of the art catalyst platforms for oxygen reduction reaction application, where catalyst lifetime is prolonged. Sandwich structure with five‐layer graphene thick (GR5/Pt/GR) showed an interesting, yet different, behavior from other sandwich structures counterparts  during ADT cycles. Through CV scans before ADT, no H as single atomically dispersed Pt catalyst possesses a fast hydrogen‐spillover rate, eliminating the observation of Hupd which is limited to ensemble Pt atomic configurations . Pt nanoparticles sandwiched between two sets of graphene sheets  were encapsulated by top graphene sheet, which acted as a shield suppressing Pt atoms dissolution in the electrolyte. During ADT, graphene may suffer from rupture due to harsh acidic and oxidizing potential, where point defects were introduced by carbon atoms removal from the graphene structure. Thicker graphene  slow down the severity of ORR, providing more barriers to reach bare‐Pt surface, which better describes the longevity of catalyst lifetime inferred from ADT analysis depicted in Figureupd up to 30K cycles, which shows graphene robustness as a shield protecting bulk Pt  from accessibility of H+, where Pt─O reduction peak charge can be attributed only to single atomically dispersed Pt atoms hopping through top layers within the graphene cap. As an agreement with the aforementioned hypothesis of Pt migration within graphene sheets, observed ECSA increase of GR3/Pt/GR sandwich structure up to 5K cycles with 7%, indicates that more single Pt atoms move within the top and bottom graphene sheets  during the electrocatalytic reaction. To validate the proposed hypotheses, ex situ Raman analysis was performed within electrochemical ADT cycles and will be discussed in the following section.The aforementioned observation is well aligned with literature reports;38 as 2.3 Single layer graphene cap suffered from significant change in graphene's nature after 1K ADT cycles, where IG/I2D increased significantly. Increase of G peak intensity (IG) relative to 2D‐peak intensity (I2D) might be due to subfolding of graphene when defects were introduced within graphene structure. In addition, Raman analysis of 2D‐peak position of GR1/Pt/GR sample after 10K cycles showed a shift of ≈77 cm−1 compared to 2D‐peak position before performing ADT. ADT is believed to introduce point defects, which was observed by an increase in D‐peak intensity, relative to G‐peak intensity. Point defect presence within graphene is thoughtful to decompress graphene samples, providing more relaxation for carbon atoms in the vicinity of the defect. This observation was validated by examining C 1s peak position from XPS which shifted toward higher binding energies, in a similar observation to Ganesan et al. Similar observation of 2D‐peak position of GR3/Pt/GR sample was depicted, however with narrower Raman shifts , as shown in Figure Graphenes’ 2D and G peak positions and relative intensity of D versus G peaks were analyzed through Raman spectroscopy. Ex situ Raman analysis was performed during ADT cycles at intervals similar to ECSA analysis reported earlier . Ex situ Raman analysis showed pronounced redshift of 2D peak for GR1/Pt/GR with increasing ADT cycles. Redshift can be attributed to relaxation of preinduced compressive strain within graphenes’ honeycomb structure.42 Sin−1 after 30K ADT cycles. In addition, D‐peak intensity increased significantly after 10K cycles, indicting presence of enormous amount of point defects within graphene's’ honeycomb structure. D‐peak intensity increased by a total of approximately fourfold after 30K ADT cycles. Moreover, graphene's’ G peak shape deviated from symmetry due to presence of D’ peak, which indicated disorder of graphene's structure. Blueshift, as described earlier, is attributed to graphene compression, however, point defects density was increased. On the other hand, 2D‐peak position of graphene was reported to be affected by the number of graphene layers. Single layer graphene reported to have 2D‐peak at lower raman shifts compared to thicker graphene sheets, where 2D‐peak position goes through a blueshift systematically with delamination of graphene . Thus, two possible scenarios can be simultaneously present: first, larger Pt atom, compared to carbon, was squeezed within graphene's defect site inducing localized compressive strain on graphene. Second, single Pt atoms intercalate between graphene layers, which increase the interplanar distances between adjacent graphene layers. Hence, five‐layer graphene behave as pseudo‐delaminated monolayers graphene, as hypothetically suggested in FigureGR5/Pt/GR sandwich structure with five‐layer graphene thick showed an opposite trend, where Raman 2D‐peak shifted toward lower Raman shifts  during ADT cycling. GR5/Pt/GR 2D‐peak position was shifted by ≈125 cmtructure.40 D‐pupd signal. As defect density increased, more Pt single atoms were stimulated to hop through graphene. Hence, single Pt atoms trapped within graphene defect site or intercalated between graphene layers at the surface vicinity acted as an active catalytic site for ORR. Concerned about prolonged catalyst durability, GR5/Pt/GR after 30K ADT cycles reserved the relative intensities of IG/I2D peaks, indicating that graphene to a great extent preserved its honeycomb structure regardless of defects presence. This is a plausible demonstration of using graphene as a catalyst‐support and a protective cap, replacing unstable, and easily‐corroded commercial carbon black , which entails boosting catalyst lifetime when tested under harsh acidic conditions.Raman analysis supports our hypothesis mentioned earlier about the electrochemical performance observed for GR5/Pt/GR with absence of Hd signal.33 As 2.4 software package, where the electronic interactions and geometric optimization were performed using Perdew–Burke–Erzenhof exchange‐correlation functional within the generalized gradient approximation. Atomic interaction energies between atomic species are obtained using a dispersive forces correction method by Grimme, and the core and valence electrons are handled by the projector augmented wave method. The plane‐wave energy cutoff of 450 eV was adopted, and the Brillouin zone was sampled with 2×2×1 Monkhorst–Pack k‐point mesh with spin polarization. Computational models as shown at Figure 1a,b, where the dimension of graphene is selected to have 12.30 Å × 12.30 Å wide to accommodate the defect and Pt clusters.Density functional theory (DFT) was used to investigate the atomic migration of Pt atom in a multiple atom defect site. We used the Vienna Ab initio simulation package (VASP)45 sof affecting electronic and/or mechanical properties of the graphene. According to Raman data presented earlier and literature support, point defect is considered to be the dominant defect type to occur in graphene during ADT. Three major possible point defect models have been studied in literature: single vacancy, double vacancy and Stone‐Wales defects. Among them, single and double vacancy defects have very similar formation energy; however, the size of a single vacancy is smaller than a Pt atom and does not allow appreciable through‐defect Pt‐hopping  at PEMFC operating conditions. Similarly, Stone‐Wales defect type, however, generated due to bonding reconfiguration, also resembles smaller size of defect for the larger Pt‐atom penetration. Therefore, in this computational study, we use a double vacancy model as an exemplary multiple atomic defects without dangling carbon atom. The model is schematically illustrated in Figure Diffusion energy barrier of metal adatoms was reported be significantly lower when the metal adatoms is trapped between two defect‐free graphene sheets compared to the energy required to liberate an adatom trapped within a graphene defect. Pristine graphene with limited amount of defect has low diffusion energy barrier, where atoms separated off Pt nanoparticles can diffuse freely. This better explains the increase of Pt ECSA for GR3/Pt/GR sample with initial 5K ADT cycles, before a critical threshold of defects‐density was reached where Pt adatoms became more prone to get trapped within defect sites.Various types of defects can be introduced on the graphene during growth or processing,16 aff In addition, our DFT analysis showed that Pt‐atoms clustering at the bottom side of the defect leads to rearrangement of atomic configuration at the defect that shift the Pt atom (Pt_D) to an off‐center position in between graphene layers. This gives an explanation of the expected catalytic degradation mechanism describing Pt migration through the graphene layers. Figure 2 carbon atoms of the graphene layers.In the same context, our DFT analysis implies that Pt clustering or agglomeration at a defect site of a graphene layer of multilayer‐thick graphene is less favorable. Figure 18 In z atomic orbitals are strongly coupled and stabilized in a giant, delocalized p bonding system. Direct contact of the graphene to the Pt surface, however, will alter the electronic structures at the interface region. By investigating the changes in electronic structure as a function of graphene sheets, we found that the interaction with Pt surface gives negligible effect on the electronic density of states (DOS) of additional graphene layers. First, electron DOS near the Fermi level are investigated as a function of graphene layers on the bare Pt(100) and Pt‐cluster‐on‐Pt (100) surfaces as exhibited at FigurePristine graphene is fairly inert with respect to chemical reactions because its pg system.49 Dir2.5 First, Pt deposition was conducted by very slow sputtering; however, clustering of atoms to form nanoparticles is expected to occur naturally on pristine graphene. To encapsulate the Pt by the top graphene sheet and make a firm contact, GR/Pt/GR sandwich was baked in air atmosphere for 90 min at 250 °C. The baking step is believed to initiate the migration of single atoms Pt to hop through point defects of graphene toward the surface. Introducing point defects in graphene could happen in oxidizing atmospheres  at 250 °C, while at elevated temperatures  a large number of defects is expected to exist. Therefore, the pathway available for Pt SAC to migrate through toward the surface is the already existing point defects within the graphene. In addition, computational analysis showed that Pt orbitals can only extend through a single layer of graphene, while thicker graphene (two or more layers) screen the accessibility of Pt orbitals to reactions intermediates (H+ or O2). Therefore, in the case of a five‐layer thick graphene cap (GR5/Pt/GR) no signal was observed for either Pt─O reduction or Hupd peaks. As the energy provided during the baking step could only be enough to migrate the Pt atoms only through the bottom‐most two to three layers, but not all the way to the 4th layer at the top of the graphene sheet. On the contrary, samples with three‐layer thick graphene showed both signals during electrochemical CV for Pt─O reduction or Hupd peaks, as it is easier for Pt atoms to reach the graphene near the surface. During ADT of GR3/Pt/GR samples set, ECSA was increased monotonically for the first 5K cycles. During that period, and under harsh acidic and oxidizing potential cycling, more point defects can be introduced within the graphene, which facilitate the migration of Pt SAC to be located adjacent the surface, where more active sites are generated. After 10K ADT cycles, the percentage of graphene defects is increased significantly and more Pt can find an easier path to the utmost graphene layer, before being washed away in the electrolyte. GR5/Pt/GR sample set showed a similar phenomenon, where the peak area of Pt─O reduction started to be observed and was monotonically increasing until 30K cycles. On the other hand, no Hupd waves was observed which confirms that the signal obtained for Pt was coming from single atoms, not cluster. Single Pt atoms has an extremely rapid hydrogen spill over rate, where the observation of Hupd on a single Pt atom is restricted to Pt atoms at the defected sites of bulk particles .GR/Pt/GR sandwich structure showed interesting electrocatalytic aptitude for ORR with 200% mass activity enhancement compared to state‐of‐the‐art commercial Pt catalyst. The most important aspect of a good electrocatalyst system is a prolonged lifetime. Sandwiched structure showed tremendous stability scaled proportionally with increasing the graphene layers thickness of the top (cap) layer facing the electrolyte. Microscopic imaging showed a consistent phenomenon of single atomically dispersed Pt surrounding the periphery of particles. Stabilizing single atoms requires a strong chemical bond  with underlying support . Our graphene does not have extrinsic doping  which is usually a defect site to stabilize SACs. Therefore, the strong interaction between the Pt SAC and graphene observed as stabile SACs under TEM imaging conditions, infer a strong chemical intimacy between Pt─C. This phenomenon of covalent‐like bond between Pt and C has been shown by our previous report.22 FirThe proposed catalyst activation/degradation mechanism was further supported by Raman analysis. 2D peak of graphene of GR5/Pt/GR was shifted to lower wave numbers, which is an indication on graphene being compressively strained. Pt atoms migrating through the point defects will be entrapped within the graphene defects, until a cluster or few atoms reach underneath to liberate it from the point defect site. This process can be observed to be stochastically challenging, where the ratio of Pt atoms entrapped in a defect site  is much larger than the time spent by other atoms , which are roaming searching for the same defect site. Pt atoms squeeze the graphene which is inducing compressive strain. Moreover, 2D peak position of graphene is sensitive to the graphene thickness. 2D peak position will shift to lower wave numbers if the graphene thickness is reduced. With Pt clustering under the defect site to surpass the activation energy barrier needed to liberate a Pt atom sitting on top of that graphene defect will start to isolate the adjacent graphene layers from one another. Increasing the percentage of defects during ADT will activate more clusters to occur. Hence, five‐layer graphene thick may behave as stacks of isolated one‐ or two‐layer thick graphene sheets.Sandwich structure provides a prolonged stability compared to bare catalysts. After 10K ADT cycles, the mass activity of GR3/Pt/GR structure was 660% compared to state‐of‐the‐art Pt catalysts supported on commercial carbon black, as shown in Figure 3In this work we demonstrate a sandwich structured electrocatalyst with Pt wrapped by two sets of graphene sheets. These GR/Pt/GR sandwich structures showed significant enhancement of catalyst stability in comparison to state‐of‐the‐art commercial Pt/carbon catalyst. The graphene cap worked as a shield preventing the loss of Pt atoms. Optimal thickness of the graphene cap was found to be three‐layer thick, where the graphene and the Pt within this GR3/Pt structure worked in unison as a hybrid catalyst for ORR. At the same time, three‐layer thick graphene showed remarkable stability, surviving approximately five‐times more Pt ECSA compared to state‐of‐the‐art commercial Pt/carbon catalyst. The catalyst degradation mechanism is thought to be through defect formation within the graphene structure, through which Pt atoms migrate toward the surface, before leaching out into the electrolyte. Yet this leaching process is tremendously slowed down by the fact that the graphene defects act as temporary traps for Pt atoms migrating to the surface. The demonstrated GR/Pt/GR sandwich structure provides a versatile material platform that can be tailored not only for electrocatalytic application, but others including environmental, biomedical, and sensing applications. The presented study provides detailed insights unraveling the origin of activity and stability degradation of metal catalysts in acidic medium of oxygen reduction reaction. This is believed to enlighten the way to design functional heterogeneous electrocatalysts with a prolonged stability.The authors declare no conflict of interest.A.A. and F.A. conceived the concept. A.A. prepared GR/Pt/GR sandwich structures, performed testing, and characterization of the samples. J.I.C. performed the DFT computational analysis. Y.D. carried out TEM experiments. A.A. analyzed all the data and primarily prepared the paper. J.I.C. and B.Z. helped in paper preparation. S.S.J. supervised DFT computational analysis. M.L. and F.A. supervised the research activities.Supporting InformationClick here for additional data file.Supplemental Movie 1Click here for additional data file.


As things stand, from a clinical point of view, there is no standardized approach for diagnosis, severity assessment, prognostic estimation, or pharmacological and non-pharmacological treatment. Although international guidelines on the diagnosis and treatment of sarcoidosis have been recently released, in both documents it was clearly stated that higher quality evidence is still urgently needed to allow recommendations with a higher level of confidence and evidence of efficacy  in a large cohort of patients, stratified according their clinical presentation: Lofgren and non-Lofgren. The results provided evidence that BAL cellular expression was directly driven by genomic expression, further highlighting the crucial role of HLA polymorphisms not only in determining the susceptibility to sarcoidosis but also in defining the clinical phenotype and endotype of the disease from the onset. In this field, the implementation of next-generation sequencing for HLA typing was conducted by Sikorova et al. to confirm the association between already described HLA polymorphisms and specific phenotypes of the disease  and heterogeneous expression of a well-recognized site of interest of sarcoidosis, such as the skin. Besides providing an extensive description, these studies further underscored the importance of a multidisciplinary approach in patients with sarcoidosis, that must be implemented by specialists with sarcoidosis-targeted expertise in order to achieve satisfying outcomes, as described by the latest guidelines , reporting innovative and intriguing insights for a better comprehension of disease pathogenesis and for the optimization of clinical assistance.PC: conceptualization and writing of the editorial. DB, AC, and CS: supervision and revision. All authors contributed to the article and approved the submitted version.


Background: Three-dimensional (3D) printing has been increasingly utilized in the healthcare sector for many applications including guiding surgical procedures, creating medical devices, and producing custom prosthetics. As personalized medicine becomes more accessible and desired, 3D printed models emerge as a potential tool in providing patient-specific education. These personalized 3D models are at the intersection of technological innovation and medical education. Our study group utilized a modified Delphi process to create a comprehensive survey tool assessing patient experience with personalized 3D models in preoperative education.Methods: A rigorous literature review was conducted of prior patient education survey tools in surgical cases across specialties involving personalized 3D printed models. Through categorization and mapping, a core study team reviewed individual questions, removed duplicates, and edited them into generalizable form. A modified Delphi process was then used to solicit feedback on question clarity and relevance from both 3D printing healthcare experts and patients to create a final survey. Results: 173 survey questions from the literature were evaluated by the core study team, yielding 31 unique questions for further review. After multiple rounds of feedback, a final survey containing 18 questions was developed. Conclusion: 3D printed models have the potential to be helpful tools in surgical patient education, and there exists a need to standardize the assessment of patient experience with these models. This survey provides a standardized, generalizable way to investigate the patient experience with personalized 3D-printed models. Establishing clear communication and understanding between physicians and patients to allow for shared decision making can be difficult in complex medical situations such as informed consent for surgery, discussing treatment plans, or clinical trials. One review found only around 50% of clinical trial patients had an adequate understanding of the elements of informed consent, including the study’s aims, risks, and benefits . More brCurrently, physicians largely employ conversation and informational documents to explain health concepts and procedures to patients in the informed consent process, occasionally using visual tools such as sketches, medical imaging, or videos for communication -8. AdvanAcross these studies, researchers used a variety of measures to evaluate patients’ experiences with personalized 3D models ranging from subjective measures of satisfaction and understanding of the diagnosis to objective measures, such as anatomical knowledge and surgical approach questions. Because there exists no standardized set of questions to assess patient experiences with 3D models in pre-surgical information exchange, it is difficult to compare overall utility across specialties. Moreover, existing surveys are often highly medicalized, specialty-specific, and only readable at an advanced level. The majority do not involve lay individuals (i.e. patients) in the process to ensure their understanding and priorities are reflected.The purpose of this research was to collect the prior body of survey questions presented to patients exposed to 3D printing in surgical applications as a basis for developing a standardized, easily understood tool. By utilizing a modified Delphi method, the survey went through rounds of anonymous feedback from patients and clinicians. This survey can be utilized to measure the benefit of patient-specific 3D printed models in patient education around surgical planning that reflects both patient and expert feedback.Study teamThe core study team consisted of five individuals including a physician, physician-scientist, social scientist, engineer/medical student, and 3D printing lab manager. The 3D printing expert team consisted of nine individuals with previous experience working with personalized 3D printed models as part of clinical and/or surgical care at the study team’s institution. The patient team consisted of 18 non-medical individuals who had previous experience with a medical procedure. Participants were recruited through email as a convenience sample. This study was reviewed and approved by the Thomas Jefferson University Institutional Review Board . Literature search A literature search was conducted using the electronic databases Pubmed, Ovid, and SCOPUS. The following keywords were searched in the title and abstract: model OR planning OR training OR education OR teaching OR assessment OR skills OR simulation AND "3D print" OR "3D printing" OR "3D Printed" OR "three-dimensional print" OR "three-dimensional printing" OR "three-dimensional printed". Articles were screened by reviewing title and abstract to determine if they met inclusion criteria and warranted full-text review. Inclusion criteria required the article be 1) published in the English language, 2) a primary article, 3) use patient-specific 3D models for patient education, and 4) administer questions assessing patient experience with 3D printed models. Articles that initially passed inclusion criteria underwent data extraction. Data extractionArticles were examined in a systematic way and data extraction included study year, specialty, total questions administered, use of objective and subjective questions, and type of rating scale used. When available, full-text questions from surveys were extracted. Categorization To understand the scope of questions being asked in prior studies and create a shorter generalizable survey, the full-text questions were assigned to one of six categories - anatomy, communication, complication, diagnosis, experience, procedure, visualization - based on the focus of the question, as determined by the core study team through consensus Table . Based oMapping To further condense the survey to a manageable length, the core study team reviewed all questions remaining after the categorization and generalization process to determine dominant themes  and grouped the questions accordingly Figure . QuestioDelphi process Round OneA modified Delphi process was used to review the list of questions generated post mapping (Appendices). For the first round of feedback, the survey was sent to a team of healthcare-associated 3D printing experts. For each question, anonymous feedback on question clarity and relevance was obtained and experts noted if it should be retained as-is, reviewed, or removed. Clarity and relevance were graded using a 5-point Likert scale and retention on a 3-point scale . Questions that received < 1.5  on retention were automatically removed. Questions that received ≥ 3.0  for relevance and clarity were kept for subsequent feedback rounds either as written, or reworded/combined with another similar question based on reviewer comments by at least two members of the core study team through consensus discussion. All other questions not meeting the above criteria underwent review by at least two members of the core study team and these questions were either reworded or combined with another similar question for inclusion in a subsequent feedback round or removed from further consideration based on reviewer comments . In a similar anonymous fashion, the patients were asked to review each question for relevancy, clarity, and retention. All questions underwent the same process as before based on the retention, relevance, and clarity scores. The resultant patient survey, both instructions written by the study team and individual questions, were evaluated for readability using the Flesch-Kincaid grade level . Figure Literature searchA total of 7141 articles were found on initial inquiry. After removing duplicates, 3705 records remained and underwent title and abstract face validity screening based on inclusion criteria. Sixty-three full text articles were selected for full content review; however upon complete review, only 30 of the 63 articles met inclusion criteria. Eight specialties were represented through these articles with orthopedic surgery being the most frequent specialty represented (n=9), followed by urology (n=6), neurosurgery (n=5), general surgery (n=3), cardiology (n=3), plastic surgery (n=2), otolaryngology (n=1), and obstetrics and gynecology (n=1).Categorization and mapping resultsA total of 173 full-text survey questions were extracted from articles. These questions were split into categories based on general focus including diagnosis (n=47), procedure (n=43), anatomy (n=24), communication (n=22), experience (n=22), complications (n=13), and visualization (n=2). Following merging of similar/duplicate questions, 53 unique generic questions were generated . Using the process of mapping questions to dominant themes, 31 key themes were identified and a unique question for each was generated from the questions in that group, yielding 31 survey questions .Modified Delphi processTen healthcare-associated 3D printing experts submitted anonymous feedback on the questions. Of the 11 anatomy questions, two questions were removed due to low retention scores (mean < 1.5). One question received an average score of ≥ 3.0 for relevancy and clarity and was retained but reworded based on expert feedback. Based on review of the remaining question for relevance and clarity scores and discussing expert comments, only one other question was retained after rewording.Of the communication questions, one question received an average score of ≥ 3.0 for relevancy and clarity and was retained as written. The second question received a score of ≥ 3.0 for relevancy but was below the cutoff for clarity so it was retained but reworded.A total of eight questions relating to procedure were reviewed by the experts. Three received an average score of ≥ 3.0 for relevancy and clarity and were retained but reworded based on expert feedback. Of the remaining procedure questions, two pairs of questions were found to be very similar and were retained but combined based on expert feedback. The final question was also retained but reworded for clarity.Ten questions relating to user experience with the patient-specific 3D models were reviewed by the experts and all 10 received an average score of ≥ 3.0 for relevance and clarity and were retained. One was kept as-is, two were combined/reworded, and seven were reworded for clarity. At the end of this round of expert feedback, the survey contained 18 questions . In the second round of the Delphi process, 18 patients submitted feedback for the survey. There were no questions that scored below a 1.5 for retention and therefore none were removed. All questions scored above 3.0 for both relevance and clarity and were retained but underwent review by at least two members of the core study team for clarity using qualitative patient feedback as a guide. Seven questions were retained as-is and 11 questions were reworded for clarity/readability. The final survey therefore contained 18 questions  with a Flesch Kincaid Grade Level of 7.8, indicating a below 8th-grade comprehension level (Table The strength of this research comes from the multi-faceted approach used to generate a specialty-agnostic, comprehensive set of survey questions that can be presented to patients. Importantly, we chose the landscape of prior survey-based research studies in orthopedic surgery, otolaryngology, general surgery, urology, and others as the foundation for our work. The questions on which our survey is based stem from the prior work of 30 other research groups across nine specialties, allowing us to take into consideration the values of each specialty.From the beginning, we prioritized creating a high-quality survey that takes into account aspects such as sentence and survey length in addition to accessible vocabulary. Given the known issue of survey fatigue and its contributing factors including survey length, topic, and question complexity, we aimed to develop a survey with less than 20 short, concise questions . To condThe Delphi method is a natural way to integrate the perspectives and values of both 3D printing experts and patients in an anonymous way into the survey . In the Patient feedback provided valuable insight for rephrasing questions and was overwhelmingly positive with most questions rated highly in both relevance and clarity. The study team found it critical to include patient perspectives while developing the questionnaire to empower patients to promote the aspects of their pre-surgical educational experience that they value most. The predominance of procedure and experience-related questions in the final survey may reflect the aspects of patient education that these personalized 3D models can address. Asking questions about patient experience may be the first step in learning about the value that patients place on having this educational tool as part of their preoperative appointment. Of course, the patient's experience with the model will be influenced by the educator using the 3D model. Additionally, the complexity of the anatomy or the time the physician spends explaining it could influence the model's ability to help patients understand. Therefore it is important to do this work to determine if patients find these personalized 3D models helpful and what they learn from the model. A well-known barrier to effective physician-patient communication is the use of language and reading levels that are too complex . While tTo our knowledge, this is the first study to describe a methodology for developing a comprehensive survey tool to assess patient perspectives on the use of 3D models in physician-patient communications for surgical applications. While previous researchers have captured patient feedback on 3D models, such feedback stems from independently generated surveys relevant to their specific specialties and surgical procedures, making it highly variable and difficult to compare across applications. Thus, a complete picture of the value of such models remains out of reach. The survey is now being utilized at our institution across specialties. In the future, the study team plans to validate the use of this survey and review the responses to determine the utility of 3D personalized models in addressing the communication barrier between clinicians and patients.The use of the Delphi process incorporated both 3D printing expert and patient feedback to better reflect the values of patient education researchers and the opinions and needs of patients. This survey can be used to gather data to improve the patient experience using personalized 3D models. Additionally, by utilizing this survey in a standardized fashion, we may determine that some model types or those used in specific specialties have more value than others. Our study did have a few limitations. Because patients involved in survey development were recruited through email as a convenience sample, selection bias may be an issue. In addition, due to the coronavirus pandemic they were unable to hold a model while giving their feedback. To fill this deficit, numerous pictures of 3D models that have been used in pre-surgical education were shown to the non-medical participants prior to collecting their feedback. Due to our decision to utilize both clinician and patient panels for feedback, we gained rounds of anonymous feedback from multiple shareholders. This meant the same group of people did not get the opportunity to evaluate the survey more than once. Personalized 3D-printed models have the potential to overcome communication barriers by providing a visual tool to allow for better patient education. The purpose of this study was to use a strong base of previously published literature in combination with a modified Delphi process to work with multiple stakeholders to develop a broadly applicable survey to measure the potential of personalized 3D models in patient education. Taking into account years of literature across many specialties ensures the longevity and generalizability of this survey, providing valuable data on this tool in patient education.


Graphene has aroused great attention due to the intriguing properties associated with its low-energy Dirac Hamiltonian. When graphene is coupled with a correlated insulating substrate, electronic states that cannot be revealed in either individual layer may emerge in a synergistic manner. Here, we theoretically study the correlated and topological states in Coulomb-coupled and gate-tunable graphene-insulator heterostructures. By electrostatically aligning the electronic bands, charge carriers transferred between graphene and the insulator can yield a long-wavelength electronic crystal at the interface, exerting a superlattice Coulomb potential on graphene and generating topologically nontrivial subbands. This coupling can further boost electron-electron interaction effects in graphene, leading to a spontaneous bandgap formation at the Dirac point and interaction-enhanced Fermi velocity. Reciprocally, the electronic crystal at the interface is substantially stabilized with the help of cooperative interlayer Coulomb coupling. We propose a number of substrate candidates for graphene to experimentally demonstrate these effects. Here, the authors theoretically predict the formation of synergistic correlated and topological states in Coulomb-coupled and gate-tunable graphene/insulator heterostructures, proposing a number of promising substrate candidates and a possible explanation for recent experimental observations in graphene/CrOCl heterostructures. K and 2. Such linear dispersions and topological properties of Dirac cones bestow various intriguing single-particle physical properties to graphene including the relativistic Landau levels, the Klein tunneling effects, and the nontrivial edge states, etc.2. Besides, low-energy Dirac fermions in graphene also exhibit distinct e-e interaction effects3, such as the interaction-enhanced Fermi velocity5, the gap opening at the charge neutrality point8, and even chiral superconductivity when the Fermi level locates at the van Hove singularity9.Graphene hosts two-dimensional (2D) massless Dirac electrons with linear dispersions and nontrivial Berry phases around two inequivalent 10, excitonic insulator12, and various complex symmetry-breaking states14. Under charge dopings, these insulating TMOs and/or TMCs may show more intriguing correlated states including unconventional superconductivity17 and long-wavelength charge density wave18.Insulating transition metal oxides (TMOs) and transition metal chalcogenides (TMCs) have also stimulated significant research interests over the past few decades due to the diverse correlated phenomena discovered in these systems such as Mott insulator19, 1T-TaS2-graphene20, and CrI3-graphene21 heterostructures, here we propose that such a scenario (of interacting Dirac fermions coupled with the correlated electrons in charge doped TMO/TMC insulators) can be realized in graphene-insulator heterostructures with gate-tunable band alignment. In this work, we show that, by virtue of the interlayer Coulomb coupling between the interacting electrons in the two layers, intriguing correlated physics that cannot be seen in either individual layer would emerge in a cooperative and synergistic manner in such band-aligned graphene-insulator heterostructures.An open question is what would happen if two types of distinct interacting many-electron systems, i.e., the interacting Dirac fermions in graphene and the correlated electrons in (slightly) charge doped TMO and/or TMC insulators, are integrated into a single platform. Especially, how the mutual couplings would affect the interacting electronic states in both systems. Inspired by recent pioneering experiments in CrOCl-graphenee-e interactions in graphene. Meanwhile, concomitant with the gap opening, the Fermi velocities around the charge neutrality point (CNP) are dramatically enhanced due to e-e interactions effects. The subbands may also possess nontrivial topological properties with non-zero valley Chern numbers that can be controlled by superlattice constant and anisotropy. Especially, we find a number of “magic lines” in the parameter space of superlattice’s constant and anisotropy, at which the Fermi velocity along one direction vanishes exactly. The subbands would acquire non-zero Chern numbers when passing through these magic lines. On the other hand, the gapped Dirac state at the CNP of graphene would further stabilize the long-wavelength EC state in the substrate by pinning the relative charge centers of the two layers in an anti-phase interlocked pattern, in order to optimize the interlayer Coulomb interactions.When Dirac points of graphene are energetically close to the band edge of the insulating substrate, charge carriers can be transferred between graphene and the substrate under the control of gate voltages due to quantum tunneling effects. This may yield a long-wavelength electronic crystal (EC) at the surface of the substrate, given that the carrier density introduced to the substrate is below a threshold value. On the one hand, the long-wavelength EC at the surface of the substrate would impose an interlayer superlattice Coulomb potential to graphene, which would generate subbands with reduced non-interacting Fermi velocity of the Dirac cone, thus trigger gap opening at the Dirac points by K and Ls = 50 Å would correspond to a carrier density ~ 7 × 1012 cm−2 transferred from graphene to the insulating substrate, which is close to the upper limit for a double-gated graphene device. Neglecting the intervalley coupling thanks to the large superlattice constant Ls (≿50 Å), we can construct an effective single-particle Hamiltonian for the continuum Dirac fermions in graphene that are coupled with a superlattice Coulomb potential  with the valley index μ = ± 1, vF is the non-interacting Fermi velocity of graphene, and Ud(r) is the background superlattice potential with the period Ud(r) = Ud(r + Ls). The superlattice of the EC is set to be rectangular, with anisotropy r = Ly/Lx and Lx,y being the superlattice constant in the x, y-direction, respectively. We denote Ls = Lx. As a result, the superlattice potential Ud(r) would fold Dirac cones into its small Brillouin zone, forming subbands and opening up a gap at the boundary of the supercell BZ, as shown in Fig. r = 1.2 (same as that of CrOCl atomic lattice) in valley K (μ = 1) with Ls = 600 Å. The energy degeneracies from folding are all lifted by Ud, whose Fourier component readsQ ≠ 0 is the reciprocal lattice vector associated with Ls, Ω0 = LxLy is the area of the primitive cell of the superlattice. The Coulomb potential Ud, screened by a dielectric constant ϵr, decays exponentially in the reciprocal space d is the distance between the substrate surface and graphene monolayer. Furthermore, the Fermi velocities near the Dirac points of the subbands are suppressed by Ud22 as clearly shown in Fig. Ls ≫ a (a = 2.46 Å is graphene’s lattice constant) is always fulfilled for low carrier density n ⪅ 1013 cm−2, with To describe the graphene-insulator heterostructure, we consider a model Hamiltonian consisted of a graphene part, an insulator substrate part, and the coupling between them (see Eqs. (5) and Supplementary Note mentclass2pt{minimmentclass2pt{minimmentclass2pt{minimmentclass2pt{minimmentclass2pt{minime-e Coulomb interactions , which depends on both the superlattice constant Ls and the background dielectric constant ϵr. Accordingly, the effective fine structure constant α = e2/) can also be tuned by Ls and ϵr, as shown in Fig. Ls, ϵr) parameter space with α > αc ≈ 0.9230, which indicates that the Dirac-semimetal phase of graphene may no longer be stable against e-e interactions within this regime according to previous theoretical study30.While it is highly desirable to open a gap at the Dirac points in graphene for the purpose of field-effect device fabrication, the superlattice potential of Eq.  alone camentclass2pt{minimUd(Q) using second-order perturbation theory, the renormalized non-interacting effective Hamiltonian for arbitrary superlattice geometry can be expressed asLs and ϵr (through Ud(Q)) for all lattice geometries. We have also calculated the effective fine-structure constants α = e2/) for both triangular and square lattices  that are closest to CNP for each valley and spin. To incorporate the influences of Coulomb interactions from the high-energy remote bands, the renormalized Fermi velocity within the low-energy subspace can be derived from the renormalization group (RG) approach38α0 = e2/(4πϵ0ℏvF) is the ratio between the Coulomb interaction energy and kinetic energy, i.e., the effective fine-structure constant of free-standing graphene, Ec is an overall energy cut-off above which the Dirac-fermion description to graphene is no longer valid. Unlike TBG39, other parameters of the effective Hamiltonian  and a slight hole doping (ν ≈ −0.003). When ν = 0, a gap can be opened up due to interaction effects  at slight hole doping ν = − 0.003, as shown in Fig. 19. We note that the EC state may be stabilized by vertical magnetic fields even when the carrier density in the substrate exceeds the zero-field threshold value43, which in turn boosts the low-field, high-temperature quantum Hall effect in the graphene layer due to the scenario discussed above.The single-particle excitation spectrum is also significantly altered by Coulomb interactions within the low-energy window, as shown in Fig. see Fig. c, e-e innds (Eq. ), but alentclass1pt{minima44, it seems to be irrelevant to the graphene-insulator heterostructures considered in the present study. For example, in CrOCl-graphene device, no magnetic hysteresis has been observed in graphene, and the measured Landau level degeneracy is still compatible with that of spin-valley degenerate Dirac cones19. Most saliently, the gap opening and the robust quantum Hall effect persist up to temperatures far above the Néel temperature of CrOCl (~14 K)19. Similarly, the magnetic proximity coupling was also reported to be negligible for CrI3-graphene heterostructure21. Therefore, compared to the power-law decaying interlayer Coulomb coupling, the exponentially decaying magnetic proximity coupling may not play an important role in such charge-transfer graphene-insulator heterostructures.Although it has been theoretically proposed that the magnetic proximity effect together with spin-orbit coupling could in principle give rise to topologically nontrivial states in grapheneThe essential results discussed above, i.e., the gap opening at CNP and the concomitant drastic enhancement of Fermi velocity, remain valid for different types of the background superlattices. Specifically, we have also performed calculations for the case of triangular superlattices, which lead to qualitatively the same conclusions, as presented in Supplementary Note 49, the low-energy subbands for graphene coupled to a rectangular superlattice potential Ud(r) with small anisotropy (r ~ 1) turn out to be topologically trivial with a compensating Berry-curvature distribution, leading to zero Chern number. This remains true even in the gapped Dirac state after including e-e interactions, as shown in Fig. 50.Different from magic-angle TBGr and the lattice size Ls of the superlattice potential Ud can make the subbands topological. For example, keeping Lx = 50 Å but with r = 3.0, both the highest valence band and the lowest conduction band acquire non-zero valley Chern numbers Cv = ± 1 . As shown in Fig. s and Xs. This additional contribution breaks the balance between positive and negative contribution of Berry curvature to Chern number, leading to non-zero valley Chern number. Such contribution stems from another accidental crossing point between the low-energy valence and conduction bands along the kx-direction through changing merely the anisotropy parameter r, as shown in Fig. Hence, it is unexpected that changing the anisotropy r from unity (with fixed Ls), the Fermi velocity in the x-direction of the valence band around the Dirac point, vx, is gradually reduced, as shown in Fig. y-direction to be intact22. Further tuning r at some point would totally flatten vx. In Fig. vx of the valence band closest to Dirac points vanishes exactly. The magic lines always come in pair as an effect of chiral (particle-hole) symmetry breaking induced by the superlattice potential. As particle-hole symmetry is broken in the energy spectrum, when vx vanishes in the valence band, the counterpart in the conduction band remains finite. The valence subband around the Dirac point has to curve upwards to create an accidental band crossing point, after that vx of the valence band becomes zero again. Therefore, a band crossing would be germinated at the Dirac point, and then move away along the kx-direction with larger r. On the one hand, the band crossing moving away from Γs is of accidental nature, which is generally avoided unless the lattice parameters are at some fine-tuned values. On the other hand, the Dirac point at Γs remains stable as protected by r at fixed Ls, the absolute value of valley Chern number of the valence subband (closest to Dirac points) increases by 1 whenever one pair of the magic lines are passed through. The positions of these magic lines also depend on the background dielectric constant ϵr since larger ϵr corresponds to weaker Fermi-velocity renormalization effect, which would shift the magic lines to larger r values. In r at fixed Ls. Such topologically nontrivial subbands with highly anisotropic Fermi velocities may provide an alternative platform to realize topological quantum matter.While increasing mentclass2pt{minim51. Then, the anisotropy of the superlattice can be artificially tuned by the dielectric patterning in the substrate. Second, for some given carrier density, the Fermi surface of the conduction  band of the substrate may be  nested, which may lead to a charge density wave (CDW) state with the nesting wavevector. For example, for CrOCl, the Fermi surfaces under different Fermi energies (above the conduction band minimum) are given in Supplementary Figure 52.We note that the anisotropic charge ordered superlattices may be realized in two ways. First, one can design a spatially modulated electrostatic potential, which has been realized in monolayer graphene by inserting a patterned dielectric superlattice between the gate and the sampleLs is also able to control the valley Chern number of the subbands. For example, with r = 3 and Ls = 600 Å, as shown in Fig. K with the two aforementioned crossing points (green circles) merely moving to Xs, the lowest conduction band turns out to be topologically trivial. This is due to two additional band crossing points (orange circles) close to the Ys-Ss line between the lowest and the second lowest conduction bands, as annotated by red dots in an orange circle in Fig. Furthermore, we find that changing The nontrivial topology must arise from the intrinsic Berry phases of the Dirac cones. Such topologically nontrivial bands are particularly surprising for our system, since the Dirac fermions are subjected to a trivial superlattice potential, which couples identically with two sublattices of graphene. Nevertheless, the nontrivial subband topology is highly tunable by changing the superlattice’s size and anisotropy  is in the form of double-gate screened Coulomb potential . The coupling between graphene and substrate is only via the long-range Coulomb potential, which is captured by Eq. , thus the exponentially decaying interlayer hopping amplitude is much weaker than the power-law-decaying interlayer Coulomb interaction. This is further confirmed by directly calculating the orbital projected band structures of a commensurate supercell of CrOCl-graphene heterostructure based on density functional theory. It turns out that the Dirac cone in such heterostructure supercell stems almost 100% from carbon pz orbitals of graphene  state (condensation energy) as a function of the carrier density We further include the interlayer Coulomb coupling between the substrate and graphene , which can be treated using perturbation theory given that the interlayer Coulomb energy is always much smaller than the sum of the intralayer Coulomb energy and kinetic energy within the relevant parameter regime  of the EC is substantially enhanced in amplitude after including the interlayer interactions, as shown by the orange diamonds in Fig. 53, the estimate of the critical density for the decoupled substrate is expected to be accurate. On the other hand, in the case of substrate coupled with graphene layer, although the interlayer Coulomb energy is estimated with Hartree-Fock approximation, the qualitative conclusion (that the EC state gets stabilized by a cooperative interlayer Coulomb coupling) is expected to be valid even in a beyond-mean-field treatment. This is because under the separable wavefunction ansatz, the interlayer Coulomb energy in the EC state is always negative (compared to that of FL state) under an optimal choice of relative charge centers, which thus always stabilizes the EC state even if the intralayer interactions are treated using beyond-mean-field approaches.On the one hand, since the condensation energy of the free 2D electron gas in the decoupled substrate is estimated using the model that accurately fits to quantum Monte Carlo data2 separated by hexagonal boron nitride55, which was also argued to be stabilized by the interlocking of the EC states in the two layers.We note that the stabilizing effect of EC is not unique to band-aligned graphene-insulator heterostructures considered in this work. In principle, it only requires the presence of another state exhibiting non-uniform charge distribution atop of the EC, such that the interlayer Coulomb energy gain would compensate for any energy cost of the long-wavelength charge modulations in the two layers. For example, remarkably robust EC state has been observed in a bilayer system consisting of two monolayer MoSe3-graphene heterostructures21, but can also be extended to various band-aligned graphene-insulator heterostructures. As along as the conduction band minimum (CBM) or valence band maximum (VBM) of the substrate is energetically close to the Dirac points of graphene, charges could be easily transferred between graphene and the substrate’s surface by gate voltages. Furthermore, it is more likely to form long-wavelength ordered state at the surface of the substrate (with slight carrier doping) if the material has large effective masses at the CBM or VBM. Meanwhile, an insulator with relatively small dielectric constant would have weaker screening effects to e-e interactions, which also favors long-wavelength ordered state at small carrier doping.The scenario discussed above is not only closely related to CrOCl-graphene and CrI3), whose CBM and VBM energy positions, dielectric constants (ϵr), effective masses at the band edges, and the corresponding Wigner-Seitz radii (rs) are listed in Table n = 1012 cm−2) are all above the threshold of forming a Wigner-crystal state (rs ≿ 31)53. In addition, the energy bands of these insulating substrate materials can be easily shifted using vertical displacement fields , MoSe2 (bilayer or thicker), and MoTe2 (bilayer or thicker) as possible candidate substrates to realize the effects discussed above. More details are given in Supplementary Note Following these guiding principles, we have performed high-throughput first principles calculations based on density functional theory for various insulating van der Waals materials. Eventually, we find twelve suitable candidate materials  of magnitude weaker than the dominant intravalley long-range Coulomb interactions in such graphene-based superlattice systems56. To model the screening effects to the e-e Coulomb interactions from the dielectric environment, we introduce the double gate screening form of Vint, whose Fourier transform is expressed as0 is the area of the superlattice’s primitive cell, ϵr is a background dielectric constant and the thickness between two gates is ds = 400 Å. Then, we initialize the Hartree-Fock loop with the initial conditions in the form of various different order parameters and obtain the converged ground state self-consistently  is the Fourier component of the charge density in the substrate. More details can be found in Supplementary Note When we consider electrons in graphene and substrate on equal footing in Eqs. (5), the routine of Hartree-Fock calculations is exactly the same. However, we need to first consider solely the substrate side. After performing unrestricted Hartree-Fock calculations, we use the ground-state charge density of EC in the substrate as input for constructing the superlattice potential. Explicitly, we need to replace Eq.  by10\doc53:c1 = − 1.106103, c3/2 = 0.814, c2 = 0.113743, c5/2 = − 1.184994, and c3 = 3.097610. These parameters are determined by fitting to quantum Monte Carlo data. The total energy for the Fermi-liquid state of 2D electron gas is given by the following model54:a0 = − 0.1925, a1 = 7.3218, a2 = 0.16008, and a3 = 3.1698. These parameters for the FL state are also determined by fitting to quantum Monte Carlo data54. The energies are given in Hartree atomic units. Then, one can extract the condensation energy for the isolated 2D electron gas in the substrate EWC − EFL, with the accuracy comparable to quantum Monte Carlo calculations.First, we start our calculations by considering solely the substrate Hamiltonian Eqs.  and 5d)5d). We c57. Then, one can integrate out the charge degrees of freedom of the substrate so that the charge density modulation characterized by the Fourier components of the charge density {ρd(Q)}  can be used as an input for the superlattice potential Ud(Q), as shown in Eq. (ρd(Q) = 2 for any reciprocal vector Q, which is equivalent to say that two (spin degenerate) charges per primitive supercell are localized in real space in a Dirac-δ-function form.Second, with the help of the separable wavefunction ansatz Eq. , we furtn in Eq. . Comparen in Eq. , this sun in Eq.  would ben in Eq.  by settie-e interactions within the graphene layer as discussed previously.Third, we perform RG-assisted unrestricted HF calculations for the interacting Dirac electrons in graphene as explained in “Methods”. If the chemical potential is at the CNP of graphene, a gap opening will be triggered by ρd(Q)} and {ρc(Q)}, based on which the interlayer Coulomb energy  can be Hgr-sub〉. Obviously, 〈Hgr-sub〉 is invariant under the global shift of the charge centers of the bilayer system, but it varies with respect to a relative charge-center shift. Therefore, by virtue of perturbation theory, optimizing the interlayer Coulomb energy amounts to find the optimal relative shift vector between the charge centers of the two layers within the degenerate ground-state manifold obtained in the previous procedures. Such perturbative treatment of Hgr-sub is justified given that the interlayer Coulomb energy is always weaker than the sum of the kinetic energy and the intralayer Coulomb energy within relevant parameter regime, as shown in Supplementary Figure Ls = 50 Å and ϵr = 4, while the intralayer Coulomb energy ~ 60 meV. More details for the perturbative calculation of interlayer Coulomb energy can be found in Supplementary Note However, the ground states are obtained so far by minimizing (mostly) the intralayer parts of the full Hamiltonian, the interlayer Coulomb interaction Eq.  is not omentclasspt{minimaFinally, we gather all the contributions from Eq.  to find It turns out that the bilayer system tends to cooperate with each other such that both the gapped Dirac state (at the CNP) of graphene and the long-wavelength charge ordered state in the substrate are substantially stabilized by the interlayer Coulomb coupling. The results are presented in Figs. We solve the Hamiltonian of the coupled bilayer system described by Eqs. (5) in the following workflow:58, as implemented in the Vienna ab initio simulation package software59. The crystal structure is fully optimized until the energy difference between two successive steps is smaller than 10−6 eV and the Hellmann-Feynman force on each atom is less than 0.01 eV ⋅ Å. The generalized gradient approximation by Perdew, Burke, and Ernzerhof is taken as the exchange-correlation potential60. As Cr is a transition metal element with localized 3d orbitals, we use the on-site Hubbard parameter U = 5.48 eV for the Cr 3d orbitals in the CrOCl bilayer and U = 3 eV for Cr 3d orbitals in the CrI3 bilayer. The so-called fully localized limit of the spin-polarized GGA+U functional is adopted as suggested by Liechtenstein and coworkers61, and the non-spherical contributions from the gradient corrections are taken into consideration. The “DFT+D2” type of vdW correction has been adopted for all multilayer calculations to properly describe the interlayer interactions62.The first principles calculations are performed with the projector augmented-wave method within the density functional theory63. We only focus at those with bulk van der Waals structures which have been previously synthesized in laboratory. This ensures that it is experimentally feasible to exfoliate few layers from their bulk sample and then stack them on graphene to form heterostructures.Our high-throughput filtering of the proper insulating substrate materials for graphene starts from the 2D materials computational databaseBy designing a dual-gated structure, we used few-layered CrOCl as a bottom dielectric while few-layered hexagonal boron nitride (h-BN) was served as top gate dielectric. The top and bottom gate voltages can then be converted into doping and displacement fields for further data analysis. Graphene, h-BN, and CrOCl flakes are mechanically exfoliated from high-quality bulk crystals. The vertical assembly of few-layered hBN, monolayer graphene, and few-layered CrOCl were made using the polymer-assisted dry-transfer method. Electron beam lithography was done using a Zeiss Sigma 300 SEM with a Raith Elphy Quantum graphic writer. Top and bottom gates as well as contacting electrodes were fabricated with an e-beam evaporator, with typical thicknesses of Ti/Au ~ 5/50 nm. Electrical transport measurements of the devices were performed using an Oxford TeslaTron 1.5 K system. Gate voltages on the as-prepared multi-terminal devices were fed by a Keithley 2400 source meter. Channel resistances were recorded in 4-probe configurations using low frequency (13.33 Hz) lock-in technique with Stanford SR830 amplifiers. The gate dependencies of channel resistances were measured at various temperatures for the extraction of thermal gaps. More details about the device configuration, measurement set-up, and sample quality can be found in Supplementary Note Supplementary InformationPeer review fileDescription of Additional Supplementary FilesSupplementary Movie 1Supplementary Movie 2Supplementary Movie 3Supplementary Movie 4Supplementary Movie 5Supplementary Movie 6


Turn-taking in everyday conversation is fast, with median latencies in corpora of conversational speech often reported to be under 300 ms. This seems like magic, given that experimental research on speech planning has shown that speakers need much more time to plan and produce even the shortest of utterances. This paper reviews how language scientists have combined linguistic analyses of conversations and experimental work to understand the skill of swift turn-taking and proposes a tentative solution to the riddle of fast turn-taking. This paper concerns the timing of speech planning in conversation. Conversation is important for our everyday lives. We use it to pass the time and bond with strangers, to conduct sales talks and selection interviews, to teach, and to derive medical diagnoses. It is where children acquire language, and, as many experienced during the covid-19 pandemic, it is something people really crave. As it is such a common and socially important type of human behavior it should be of central interest to cognitive and language scientists. Studying conversation is also important for practical reasons. Though it is typically experienced as effortless, conversation can become taxing in persons with speech or language impairments, for instance after a stroke, in persons with hearing loss and non-native speakers of a language. To support such individuals, diagnosis of their difficulties is required, which presupposes a clear view of typical conversation. Finally, conversations occur not only face-to-face, but also in remote contexts , short phrases (“no milk!”), or longer utterances. In addition, there are backchannels, such as “uhu” or “yeah”, which listeners use to encourage their partners to continue their turns and which are often not classified as turns themselves e.g., , 2016. TThe fourth property, which is most central for this paper, is the temporal coordination between turns. Most of the time only one person talks and the speakers’ turns follow each other promptly. Levinson and Torreira  note thaThe tight coordination of turns in content and timing shows that speakers generally succeed in planning and producing a turn very shortly after the end of the preceding turn. This is remarkable because utterance planning is not instantaneous but requires substantial amounts of time. For instance, in lab experiments participants typically need 600 ms to 800 ms to name a line drawing of a common object e.g., , and preThe gaps between turns appear to be mysteriously short only as long as one assumes that comprehension and production of turns occur strictly in sequence; i.e. that a person first listens to all of the interlocutor’s turn and then begins to plan a response. The mystery is solved if listening and response planning are allowed to overlap in time, i.e. if speakers begin to plan a turn before the end of the partner’s turn. For many turn sequences, this is plausible. For instance, in a café a customer might not need to hear much more than “What can …?” to know that the barista is ready to take the order and to respond accordingly.Levinson and Torreira  proposedLevinson and Torreira’s model is important for the language sciences because it bridges between descriptive linguistic work on conversation and lab-based psycholinguistic work. This is because it explains the coordination between turns in time and content by reference to specific cognitive processes: early recognition of gist and speech act, prediction of ends of turns, and early response preparation. The model can be evaluated by assessing, first, whether these processes indeed take place and, second, whether they lead to short gaps between turns. Conducting such a research program is not straightforward because most experimental paradigms require participants to carry out specific tasks at specific times and therefore cannot be used while speakers are engaged in spontaneous conversation. However, one can ask whether the central claims of the model are consistent with laboratory findings and current theories of speech processing and planning. This question is discussed in the current and the next section of this paper.Two key assumptions concern listening in conversation. The first one is that listeners can grasp the partner’s meaning before the end of their utterance. This assumption is consistent with a strong body of evidence showing that speech processing is highly incremental and opportunistic, with all available evidence immediately being used to infer the meaning and to predict upcoming parts of the utterance . In addiThe second claim is that listeners predict ends of turns and launch prepared responses in anticipation rather than in response to them. This claim is consistent with the strong evidence for prediction during language processing already mentioned above and with specific evidence concerning listeners’ ability to predict ends of turns. For instance, Corps, Gambi, and Pickering  showed tThe third claim of Levinson and Torreira’s model concerns the timing of speech planning: Listeners, aka next speakers, begin to plan their utterances as soon as they have enough information to do so. This claim implies that listening and speech planning often occur at the same time. It is this head-start in speech planning relative to the ends of turns that, according to this account, leads to the short gaps between turns.But can speakers prepare utterances while listening? And does such early preparation for speaking indeed contribute to short gaps between turns? This is not self-evident, as one might expect listening and speech planning to interfere with each other. However, several experiments have shown that speech planning during listening is indeed possible and that it facilitates fast responding. The first relevant experiment was carried out by Bögels, Magyari, and Levinson . The parStudies using related paradigms found compatible pattern of results e.g., . For insIt is, however, worth noting that upcoming speakers do not necessarily begin to plan utterances as early as possible. For instance, in a study by Sjerps and Meyer , particiThe fourth claim of Levinson and Torreira’s model is the distinction between response planning and launching: Speakers begin to prepare a response to their partner as soon as possible, but only launch it shortly before the anticipated end of the partner’s turn. This proposal is consistent with a large body of experimental work using delayed naming tasks, which has shown that speakers can indeed generate speech plans internally, retain them in working memory, and produce them upon presentation of a response cue  and to avoid hesitations and repairs. Even under those circumstances, participants often choose not to plan the entire utterance but only a first chunk, often corresponding to one or two words, before beginning to speak. This strategy can lead to disfluencies or pauses after the first chunk e.g., . In convRegardless of the mechanisms and strategies that may support fast responding in conversation, speakers always need some time to hear and understand at least the beginning of the partner’s utterance , to decide what to say, to retrieve an appropriate word or phrase as an answer (“Not yet.”), and to launch it. As discussed above, speakers need to have a complete speech plan for the beginning of their utterance to respond to a partner within a few hundred milliseconds. Given laboratory results concerning the time needed for speech planning, it is unlikely that a complete speech plan, even for a short utterance, can be created in much less than a second. This means that turns have to be at least about 800 ms long to receive responses with gaps of 200 ms.How long are turns in conversation? In the published literature, there is surprisingly little information about turn durations. There are many phonetic studies of conversational speech where information about utterance durations must have been gathered but is not reported, presumably because this information was not of interest to the researchers. Levinson  suggestsTo add to this literature, Corps, Knudsen, and Meyer  set out This result is puzzling. How can the gaps between the speakers’ utterances be so short when the current speaker’s utterance is too short to allow the next speaker to prepare a response? Further analyses of the corpus showed that many of the utterances that were automatically labelled as turns were not complete turns, but only parts of turns. This situation most commonly arose when the speakers talked at the same time, as is illustrated in (1). Referring to a bar discussed earlier, Speaker B says “Ok, da war aber halt nichts los.” , and the other speaker simultaneously says “Da beim Chinesen nebendran, gell? . In the transcript, the two parallel utterances are aligned word-by-word and rendered, incorrectly, as an exchange of one- or two-word turns.To assess how often this situation arose, Corps and colleagues categorized each automatically defined segment as a self-continuation or a different type of segment. Self-continuations were defined purely in syntactic and lexical terms, e.g. when a segment missed a verb phrase that was provided in the next segment by the same speaker, or by the use of pronouns referring to a preceding segment. The use of these stringent criteria allowed for transparent and replicable coding of the segments. Corps and colleagues found that 24% of the segments were self-continuations. For the purpose of determining the length of turns self-continuations should be combined with the preceding segment by the same speaker. When this was done, the average turn duration rose to 6.0 seconds, and the median to 3.4 seconds. The gap between turns remained close to zero, with a mean of –.09 seconds and a median of –0.02 seconds. Thus, in contrast to the initial impression based on the automatic parsing of the utterances, these results suggest that the speakers in this conversation usually did have enough time to prepare a turn while their partner was talking. It is important to stress that the above turn durations only concern the relatively small German corpus analyzed by Corps and colleagues. Further work is needed to obtain a better estimate of the proportions of self-continuations and the durations of turns in informal conversation.The analyses carried out by Corps and colleagues also showed that the speakers often did not use all of the time afforded by the partner’s utterance to plan their own turn and launch it shortly before the end of the partner’s turn. Instead, they often began to speak much earlier. As noted already, 24% of the segments stemmed from episodes of parallel talk, and 9% of the turns were fully embedded in longer turns, i.e. began after and ended before the end of a partner’s turn. Why do speakers talk at the same time? In the linguistic literature parallel talk has often been linked to premature turn-taking e.g., : A speakParallel talk is similar to the use of backchannels, which are utterances such as “uhu” or “ehem”. In the German corpus analyzed by Corps and colleagues, 23% of the segments were backchannels. They are often not considered to be turns themselves, but as encouragement to the current speaker to continue their narrative or elaborate on what they said before e.g., , 2016. IThe goals of this paper were, first, to illustrate how experimental psycholinguistics and linguistic approaches to language can be combined to understand how language is used in conversation, and second, to propose and motivate a specific account of rapid turn-taking. To turn to the first goal, Levinson and Torreira’s  model isThe second goal was to address the question how speakers manage to respond to each other almost instantaneously. We offer two complimentary answers. The first answer was already proposed by Levinson and Torreira . Gaps beThe second answer is that speakers in conversation often do not respond directly, segment-by-segment, to the content just expressed by their partner. Instead one person talks, while the other provides backchannels, or the speakers develop their turns in parallel. In parallel talk, speakers engage in linguistic dual-tasking but the need to respond rapidly and appropriately to the partner’s utterance does not arise. Parallel talk may occur when a speaker responds to the content expressed early in the partner’s turn, perhaps anticipating that the turn would end sooner than it actually did.The two answers are related. Both imply that listeners quickly grasp the meaning of the partner’s turn and begin to formulate a response. “Neat” sequential turn-taking, with one speaker responding close to the end of the other’s turn, occurs when the second speaker estimates correctly when the partner’s turn will end and times their fully prepared utterance to coincide closely with that event. As discussed above, achieving such tight coordination of turns is no mean feat and requires accurate prediction of turn ends, in parallel with response planning and timely launching of the prepared utterance, as described in Levinson and Torreira’s model. In parallel talk, the upcoming speaker also plans a response during the interlocutor’s turn, but times it to begin well before the end of the partner’s turn, either misjudging how long the partner will continue talking or simply not taking this into account. Talking during concurrent speech input requires a speaker to divide their attention between listening and speech planning, and the selection of words for speaking may be hampered because of interference from the spoken words. This may lead to hesitant speech featuring silences and filled pauses. Speakers might find it difficult to predict ends of turns in hesitant speech, which may lead to further parallel talk. This is how long stretches of parallel talk may arise.Further empirical and theoretical work is needed to flesh out and test this proposal and, more generally, understand how participants in conversation coordinate their utterances in time and content. The model proposed by Levinson and Torreira  has stimIn addition, very little is known about the way speech comprehension and speech planning processes interface with motivational processes and social cognition, which likely strongly shape both the content and the timing of conversations. Here, an important open question is why casual conversation adheres to tight time constraints in the first place. Why do people prefer to respond swiftly to each other, even though this affects the fluency and well-formedness of their utterance? And why do they talk in parallel even though this must be effortful and may affect mutual understanding? As mentioned earlier, swift responding has been linked to enjoyment and a feeling of social connection, i.e. of being heard and understood. This is plausible, but one might wonder why a feeling of social connection is linked to fast, rather than slow (and thoughtful) responding. It has also been proposed that conversation is a form of joint action, which requires well-coordinated responses e.g., . The feeTo gain a better understanding of these issues and the cognitive processes underlying conversation, corpus analyses must be combined with experimental work. In the corpus work, researchers need to use or generate richly annotated corpora, where turns and sub-turn units are tagged, and where gaps between turns can be distinguished from other inter-speaker gaps. As illustrated above, transcripts based solely on phonetic information will often not suffice to identify the beginnings and ends of turns. Suitable corpora have been generated in different labs, for instance by Kendrick and Holler , RobertsRichly annotated multi-modal corpora provide descriptions of the interlocutors’ behaviour. They reveal what the speakers say, which gestures they make, and when they do so. They also reveal how the speakers’ utterances are related in time and content. By their very nature, spontaneous conversations offer researchers no control over the participants’ behavior, and so analyses of conversational speech are not sufficient for testing processing theories of speaking and listening in conversation. Therefore, corpus analyses need to go hand-in-hand with experimental work. Here, the challenge is to design experimental paradigms that allow for stringent control of the variables of interest in settings that optimally approximate natural conversation.