N = 200 university students who were randomly assigned to (a) adaptive testing, (b) non-adaptive testing, or note-taking (c) without or (d) with focus guidance. In an initial study phase, all participants watched an e-lecture. Afterward, they processed its content according to their assigned conditions. One week later, all learners took a posttest. As main results, we found that adaptive testing yielded higher learning outcomes than non-adaptive testing. These benefits were mediated by the adaptive learners’ higher testing performance and lower perceived cognitive demand during testing. Furthermore, we found that both testing groups outperformed the note-taking groups. Jointly, our results show that the benefits of testing can be enhanced by adapting the sequence of test questions to learners’ knowledge and that testing can be more effective than note-taking.Testing is a well-established desirable difficulty. Yet there are still some open issues regarding the benefits of testing that need to be addressed. First, the possibility to increase its benefits by adapting the sequence of test questions to the learners’ level of knowledge has scarcely been explored. In view of theories that emphasize the benefits of adapting learning tasks to learner knowledge, it is reasonable to assume that the common practice of providing all learners with the same test questions is not optimal. Second, it is an open question as to whether the testing effect prevails if stronger control conditions than the typical restudy condition are used. We addressed these issues in an experiment with  Testing is a well-established desirable difficulty. Making the learning process more difficult by providing learners with test questions that require retrieval from memory after an initial study phase has been shown to foster retention and transfer . HoweverFirst, it is unclear whether the full potential of testing has yet been exploited. In real educational settings such as regular high-school lessons or university lectures, testing is often implemented by having all learners answer the same pre-set sequence of specific test questions e.g., . HoweverSecond, a recent discussion regarding which control condition the benefits of testing should be compared to has gained momentum. Specifically, it has been argued that the commonly used restudy control condition might be too weak because restudy has relatively low utility as a learning method . Comparitesting conditions) or were assigned to one out of two note-taking conditions. The note-taking conditions differed in whether learners were guided to focus on the same learning content items as the learners who received test questions. Learning outcomes measured after 1 week delay served as the main dependent variable.To address these issues, we conducted an experiment in which all learners watched an e-lecture in an initial study phase. As a follow-up learning task, the learners received either an adaptive or a non-adaptive sequence of specific test questions . By contrast, in specific testing, learners reply to specific test questions that are directed at certain but not necessarily all learning content items.There are two basic ways to implement testing with desirable effects: (a) the provision of free recall prompts e.g.,  or (b) tpractice quizzing, e.g., right test questions has received scarce attention so far.In view of the fact that in real educational settings, such as high-school lessons or university lectures, the desirable difficulty of testing is frequently implemented via the provision of specific test questions  to an adaptive approach in which test question complexity was determined by the learner’s knowledge (adaptive testing). The learners’ level of knowledge was diagnosed by how they answered a previous test question that had covered the same content.In the present study we compared a non-adaptive implementation of specific testing that proceeded from simple to complex higher-level test questions without taking learners’ knowledge into account  and if the effects between adaptive and non-adaptive testing would be driven by differences between the conditions concerning (a) the performance on the test questions and (b) the perceived cognitive demand during testing (research question 2). A secondary goal of our study was to contribute to the research that analyzes the benefits of testing in comparison to stronger control conditions than restudy. Specifically, we wanted to know whether testing that is implemented via the provision of specific test questions would be more beneficial than both note-taking and focused note-taking (research question 3).f = 0.25; d = 0.50), we conducted an a priori power analysis using G∗Power 3.1.9.2 . Due to non-compliance, n = 13 participants had to be excluded from analysis, resulting in a final sample size of N = 187. The learners were randomly assigned to one condition of a one factorial between-subjects design that comprised four conditions. In an initial study phase, all learners watched an e-lecture. The experimental manipulation took place in the learning phase that occurred after the initial study phase. In this learning phase (80 min), the participants learned either by (a) adaptive testing, (b) non-adaptive testing, (c) note-taking, or (d) focused note-taking. The learners received €40 for their participation. The experiment was approved by the Ethics Committee of Bielefeld University, Germany (No. 2017 – 049) and written informed consent was obtained from all participants.In light of the notions that the potential benefits of (a) adaptive testing in comparison to non-adaptive testing and (b) testing in comparison to (focused) note-taking would be educationally relevant if they would be of at least medium size  test questions required the learners to reproduce main content from the e-lecture one subtopic at a time. Depending on the social norm, this was either the definition of the norm or a technique that was based on it, and their respective conditions .one social norm or technique  Which of those factors are met regarding Anja and her senior students, (b) which factors are not met, and (c) for which factors is it not possible to reach a conclusion from the information provided in the text? Justify your answers by providing short explanations.”). On Level 3, the test questions required learners to simultaneously consider two social norms or techniques at a time ; in the Level 4 test questions, all three social norms or techniques were intertwined (reciprocity × commitment × obedience). In both testing conditions, the subtopics alternated between Levels 1, 2, and 3, so that the same subtopic(s) would not be presented back to back.In the more complex higher-level test questions , the learners were asked to analyze a situation described at the beginning of the test question in which the respective social norms or techniques were applied. The Level 2 test questions asked for the analysis of the situation with respect to only The shared view that those with legitimate authority should be obeyed contains the idea units shared view, people with legitimate authority, and should be obeyed). The participants then had to score their answers by evaluating if the idea units were fully, partially, or not at all included in their answer. This procedure was based on the feedback method used in a study by After each test question, the participants evaluated their responses by using a feedback form  and then three Level 2 questions , followed by three Level 3 questions . When they reached the final level, the participants then received Level 4 test questions for the remainder of the learning phase.tested content; see Section “Testing”). The learners were asked to focus their learning efforts on these slides . Content on slides that did not include red frames  was semantically associated with the focused tested content but was not crucial for its understanding. Untested content could therefore potentially serve as a retrieval cue for the respective tested content. For instance, untested content slides provided a definition of the norm (which helped to understand the mechanics of a technique), a study that illustrated a norm, and/or provided information on the relevance of the norm in everyday life or for historic events . The remaining nine questions tested the participants’ knowledge regarding content at which the test questions had not been directed . The questions on the tested content can be further differentiated into the subscales familiar questions, near transfer questions, and generation questions. The familiar questions were questions that the participants of the testing conditions had already answered on Level 1 in the learning phase (posttest Cronbach’s α = 0.68). The near transfer questions were structurally similar to the Level 2, 3, and 4 test questions, but related to different situations than the test questions (posttest Cronbach’s α = 0.76). The generation questions asked the learners to generate their own example situations that illustrated the social norm or technique. There was one generation question for each of the three social norms or techniques (posttest Cronbach’s α = 0.62).We created a pretest to measure the participants’ prior knowledge on the topic of social norms before the initial learning phase and a posttest to measure learning outcomes thereafter. Both tests contained the same 22 questions. Thirteen of the questions covered the content at which the test questions in the learning phase had been focused . The final scores were obtained by calculating proportion scores of the theoretical maximum score for each question. The proportion scores were then aggregated to determine the scores regarding the aforementioned subscales tested content, familiar questions, near transfer questions, generation questions, untested content as well as the subscores for the four question levels by averaging the respective question scores. By using this procedure, each question was given the same weight .Performance on the test questions was obtained by using the participants’ self-scores (see Testing) and calculating proportion scores of the theoretical maximum score for each test question. These proportion scores were then averaged for each test question level which led to all test questions having equal weight .ery low degree of cognitive demand to (11) very high degree of cognitive demand. These scores were then averaged for each test question level. As this measure was linked to the test questions, it was only restricted to the testing conditions.In the testing conditions, the perceived cognitive demand was assessed for every test question before the learners received the feedback. For this purpose, we asked the learners, “How demanding did you find the question?” using an 11-point Likert scale ranked from (1) v1.In all conditions, the participants worked individually in a digital learning environment on computers in a laboratory. First, all learners took the pretest. After finishing the pretest, they were asked to watch the e-lecture attentively because they would be given questions on its content afterward . Following the e-lecture, the participants had to take a 5-min break before the experiment resumed. Afterward, they received instructions for the subsequent learning phase according to their condition and processed the e-lecture content accordingly. The learning phase had a duration of 80 min in all conditions. At the end, the participants were informed that the experiment would continue in one week’s time and were asked not to deal with the subject of social norms until then. In the second experimental session, the participants took the posttestp-values for all analyses). For F tests, we report partial η2 as the effect size measure. In line with t-tests, we report Cohen’s d qualifying values of approximately 0.20 as small effects, values of approximately 0.50 as medium effects and values of 0.80 as large effects.An α-level of 0.05 was used for all statistical analyses (we report both two-tailed and one-tailed M = 4.1% of the maximum attainable score). Surprisingly, there was a statistically significant difference between the groups regarding prior knowledge, F = 11.79, p < 0.001, F = 0.38, p = 0.767, To test whether the random assignment resulted in comparable groups, we compared the learners’ pretest scores and grade point average. Overall, pretest scores were very low  = 7.67, p = 0.007, p = 0.004). To enable a more differentiated interpretation of the results, we also analyzed the subscales of the posttest  = 10.36, p = 0.002, p = 0.001) but not on untested content questions, F = 0.56, p = 0.457, p = 0.229). Furthermore, within the tested content questions, the adaptive group outperformed the non-adaptive group regarding performance on familiar questions, F = 13.82, p < 0.001, p < 0.001), near transfer, F = 5.65, p = 0.020, p = 0.010), and generation questions, F = 3.14, p = 0.080, p = 0.040). The tested content questions were also differentiated with respect to question level. The adaptive group performed better on Levels 1 and 2 posttest questions, F = 13.82, p < 0.001, p < 0.001), and F = 18.59, p < 0.001, p < 0.001). No significant effects were found for Levels 3 and 4, F = 0.19, p = 0.667, p = 0.334), and F = 0.03, p = 0.862, p = 0.431).In research question 1, we were interested in whether adaptive testing would yield higher learning outcomes than non-adaptive testing. An ANCOVA revealed a statistically significant effect on the overall posttest score, with the adaptive group scoring higher than the non-adaptive group, t-tests to analyze the extent to which the groups differed on these learning process measures  = 4.30, p < 0.001, d = 0.91 (one-tailed: p < 0.001). This overall superiority was driven by the fact that the adaptive learners scored higher on Levels 1, 2, and 4 test questions, t(76.32) = 5.61, p < 0.001, d = 1.14 (one-tailed: p < 0.001), t(90) = 7.37, p < 0.001, d = 1.56 (one-tailed: p < 0.001), and t(48) = 2.53, p = 0.015, d = 1.35 (one-tailed: p = 0.008). There was no statistically significant difference regarding performance on Level 3 test questions, t(71) = 0.69, p = 0.495, d = 0.17 (one-tailed: p = 0.248).Regarding performance on the test questions, we found that overall the adaptive group performed significantly better, t(90) = 6.87, p < 0.001, d = 1.45 (one-tailed: p < 0.001). This result was mainly due to the non-adaptive testing group reporting higher cognitive demands on Levels 1 and 2, t(90) = 4.27, p < 0.001, d = 0.90 (one-tailed: p < 0.001), and t(90) = 7.28, p < 0.001, d = 1.54 (one-tailed: p < 0.001). The non-adaptive learners also indicated higher cognitive demands on Levels 3 and 4, t(36.40) = 3.13, p = 0.003, d = 0.87 (one-tailed: p = 0.002), and t(48) = 1.80, p = 0.078, d = 0.96, (one-tailed: p = 0.039). However, due to the relatively low number of adaptive learners who reached Levels 3 and 4, these findings have to be interpreted cautiously.Analyses of the cognitive demand of the test questions revealed that, on average, the non-adaptive testing group rated the test questions as more cognitively demanding than the adaptive testing group, t(64.36) = 5.64, p < 0.001, d = 1.23 (one-tailed: p < 0.001). This effect was driven by the fact that the adaptive testing group received a statistically significant higher number of Levels 1 and 2 test questions, t(42.00) = 10.60, p < 0.001, d = 2.39 (one-tailed: p < 0.001) and t(42.00) = 8.01, p < 0.001, d = 1.81 (one-tailed: p < 0.001). Regarding Level 3 test questions, there was no statistically significant difference between the groups, t(42.50) = 0.97, p = 0.337, d = 0.22 (one-tailed: p = 0.169). However, the non-adaptive group answered more Level 4 questions, t(53.93) = 8.86, p < 0.001, d = 1.76 (one-tailed: p < 0.001). In view of these large differences regarding the number of received test questions, we included this learning process measure as a further potential mediator in our analyses.For explorative purposes, we also analyzed the number of test questions that were answered in the learning phase. We found that the adaptive testing group answered significantly more test questions than the non-adaptive group, To test whether the reported significant differences regarding learning processes mediated the superiority of the adaptive testing group in terms of learning outcomes, we conducted mediation analyses in the second step of our analysis. This was done by calculating a simultaneous multiple mediation model in which the dependent variable (the posttest scores) was regressed on the independent variable condition as well as the potential mediators performance on the test questions, the cognitive demand of the test questions and the number of test questions that were answered in the learning phase. Pretest scores were included as a covariate. By using the SPSS macro PROCESS see , we testWith respect to performance on the Level 1 posttest questions, we found a statistically significant indirect effect via the performance on the Level 1 test questions . The indirect effects via the number of test questions on Level 1 and the cognitive demand of the Level 1 test questions did not become statistically significant . Hence, the superiority of the adaptive testing group regarding performance on the Level 1 posttest questions was mediated by their higher performance on the Level 1 test questions but not by the number or the cognitive demand of test questions. For the Level 2 posttest questions, we found statistically significant indirect effects via the performance on the Level 2 test questions and the cognitive demands of the Level 2 test questions . We did not find a statistically significant indirect effect via the number of Level 2 test questions . These analyses indicate that the superior performance of the learners in the adaptive testing group on the Level 2 posttest questions was mediated via their higher performance on the Level 2 test questions as well as the lower cognitive demand they experienced while answering them.F = 0.02, p = 0.901, p = 0.451), tested content, F = 0.08, p = 0.777, p = 0.389), and untested content, F = 0.15, p = 0.702, p = 0.351; see Table In research question 3, we were interested whether testing would yield superior learning results than both note-taking and focused note-taking. There were no statistically significant differences between the two note-taking groups with respect to the overall score, F,92 = 0.0F = 0.14, p = 0.713, p = 0.357), but only on tested content, F = 14.55, p < 0.001, p < 0.001). However, the note-taking groups showed better results than the non-adaptive testing group regarding untested content, F = 16.25, p < 0.001, p < .001). For the analysis regarding the adaptive testing group, we assigned the contrast weights 0.5 for note-taking, 0.5 for focused note-taking and -1 for adaptive testing. The adaptive testing group outperformed the note-taking groups on the overall score F = 10.04, p = 0.002, p = 0.001) as well as on tested content, F = 53.68, p < 0.001, p < 0.001), but showed a weaker performance than the note-taking groups on untested content, F = 9.54, p = 0.002, p = 0.001).To compare each of the two testing groups with the two note-taking groups, we used contrast analysis. Using contrast analysis is recommended by Our study contributes to the literature on the desirable difficulty of testing by casting light on two moderating aspects of the benefits of testing. Firstly, our results show that adaptation is a promising means to optimize specific testing. Increasing the fit between the learners’ current state of knowledge and the test questions increased the benefits of testing compared to non-adaptive testing. Therefore, the desirability of testing evidently can be improved by providing adaptive test questions. Secondly, our results indicate that non-adaptive specific testing leads to better learning of tested content than note-taking, even if the latter is supported by focus guides. These findings strengthen the case for implementing specific testing in real educational environments.zone of desired effects within educational research . Nearly all tested content effect sizes were of medium scale which puts the adaptation effects in the ng e.g., . Furtherng e.g., . Jointlyall learning outcome measures. First, the testing groups did not differ in terms of performance on the untested content questions. As both the adaptive and non-adaptive group did not have access to untested content items during their learning phase, this finding is not surprising. Second, regarding the tested content questions, no differences between the scores of the two testing groups on Levels 3 and 4 posttest questions could be found. At first glance, one explanation for this pattern of results could be that the adaptation mechanism caused the learners in the adaptive group to remain on Levels 1 and 2, where they answered significantly more test questions than the non-adaptive group. Due to limited learning time, this led to fewer learners reaching and answering the Levels 3 and 4 test questions. It would therefore be reasonable to attribute the results for Levels 3 and 4 posttest questions to the fact that many learners did not answer any Levels 3 or 4 test questions at all during the learning phase. However, our mediation analyses contradict this explanation. For performance on both the Levels 1 and 2 posttest questions, we found that not the number of test questions per se, but the learners’ performance on these test questions mediated the benefits of testing. This pattern of results suggests that the lack of superiority of the adaptive group over the non-adaptive group regarding performance on the Level 3 and Level 4 posttest questions should be due to a lack of significant differences between the testing groups in terms of their performance on both levels. However, this was only the case for Level 3 test questions. For the Level 4 test questions, we found a significant difference in favor of the adaptive learners. This surprising finding likely is due to the fact that only a small sample of participants (4 out of 43) of the adaptive testing group actually reached the Level 4 test questions in the learning phase, compared to nearly all participants (46 out of 49) in the non-adaptive testing group. Consequently, the superiority of the adaptive group regarding performance on Level 4 test questions should be interpreted very cautiously. Surprisingly, even though most of the learners in the adaptive group did not reach the Level 4 test questions (n = 19 learners did not even reach Level 3), they were not outperformed by the non-adaptive group regarding Levels 3 and 4 posttest performances. This suggests that the adaptive testing group had profited from working on the lower level test questions on Levels 1 and 2 to such an extent that it evened out the lack of practice on Levels 3 and 4.However, it is important to highlight that this conclusive pattern of results was not found in terms of Jointly, our results regarding the comparison of adaptive and non-adaptive testing are closely in line with the general notion that learning outcomes can be improved by matching the complexity of learning tasks to the learner’s current state of knowledge see . HoweverThe second limitation concerns the cut-off values that were used to determine the sequence of the adaptive test questions. The cut-off values that were used led to the participants in the adaptive condition to remain on the Levels 1 and 2 test questions. Additionally, as mentioned above, only a few participants in this condition reached the Level 4 test questions. This raises the question whether the cut-off value used to advance to a higher level, namely 85%, was too high and blocked the participants from advancing to the higher-level questions. Although this might have been the case, it may have actually been advantageous to a certain extent. Third, the adaptation mechanism as implemented in this study is not the only conceivable mechanism. It is reasonable to assume that learners who perform well on learning tasks such as test questions still might perceive themselves as being overloaded, which could affect their motivation and capacity to invest effort in subsequent (and more complex) learning tasks. In this case, adapting tasks not only to learners’ performance but also to their perceptions of the current cognitive demands could be beneficial. Therefore, more subjective measures such as the perceived difficulty of questions or the perceived cognitive demand could be a basis of adaptation that might be worth addressing in future research.In previous research, testing has been mainly compared to restudy. However, due to the low educational utility of restudy, the true educational value of testing has been the subject of discussion in recent years. Our results show that the testing effect prevails against the presumably stronger control condition note-taking. This was also shown when note-taking was accompanied by focusing guides that aimed at canceling out any potential focusing advantages inherent to specific testing.It should be highlighted, however, that the non-adaptive testing group did not outperform the note-taking groups on the overall posttest score. One explanation for this finding is that the note-taking groups had a clear advantage on untested content due to the way the experiment was set up, presumably canceling out the advantage on tested content by the non-adapted testing group. Only tested content was subject of both the testing and the note-taking conditions’ learning phase and thus only tested content enables a true comparison between testing and note-taking. Regarding tested content, the non-adaptive testing group clearly outperformed the note-taking groups.The finding that the testing groups showed higher scores on tested content even though they performed worse on untested content than the note-taking groups indicates that although tested content and untested content was semantically related (see Note-Taking), acquiring knowledge of tested content did not require knowledge of untested content. It can therefore be assumed that the content which was designed as untested content was indeed not crucial for succeeding in understanding the tested content.The results regarding untested content also provide evidence of note-taking’s utility as a learning method. After the initial learning phase, the participants in the testing conditions did not have access to the content that was the subject of the untested content questions. Therefore, the testing conditions equaled no-study conditions regarding the untested content. The two note-taking conditions performed substantially better on the untested content than each of the two testing conditions (medium effect). This underlines the assumption that note-taking is a suitable control condition .d = 0.63 , it can be concluded that prompting learners to focus on specific content items did not have an effect on their learning outcomes. This calls into question the extent to which our implementation of the focus guides increased focus on the intended content. Our focus guides were visual markers that were designed to serve as a reminder for the learner to focus his/her attention on the relevant content. By contrast, the focus guides discussed in the literature, such as questions on targeted segments and elaborative investigation questions, are often designed to not only focus learners’ attention on certain content items but also elicit some kind of elaboration or retrieval  to the test questions, which warrants caution when comparing the learning outcomes of the testing and note-taking conditions. This may have led to an advantage for the testing conditions because they were already familiar with the question type. This could therefore partly explain the testing effect that was found regarding the tested content. However, the participants in the testing conditions also fared better on the generation questions (which were only used in the pre- and posttests) compared to the learners in the note-taking conditions. This shows that the testing effect found, albeit small, is not solely based on familiarity with the questions in the posttest.In addition to specific limitations concerning the interpretation of the results regarding our research questions discussed above, attention should also be paid to two aspects of the experimental setup. First, it is important to highlight that our findings only apply to open-ended test questions. Research on testing suggests that close-ended questions such as multiple or single choice questions e.g.,  would haSecond, it is worth mentioning that in the present study we implemented testing in a closed-book format. That is, the learners were not provided with the opportunity to go back and watch parts of the e-lecture again while responding to the test questions. Even though a closed-book format is the usual format of testing, only very little research has shown it to be more effective than open-book testing e.g., . Some stA third fruitful avenue for further research could be to investigate the effects of the type of testing  in a more systematic manner. For instance, it can be assumed that specific testing and general testing lead to different mnemonic effects. Both specific and general testing rely on retrieval, but they may differ in terms of (the amount of) elaboration processes triggered by the learning task. During general testing, it is up to the learners how much elaboration takes place and which content items are covered; during specific testing, dependent on the type of specific test questions , learners are forced to retrieve and elaborate on specific content items. Hence, in future studies it could be valuable to compare specific and general testing in terms of learning outcomes as well as learning processes. Regarding the latter, the number of covered content items and the degree of elaboration should be monitored. Moreover, in these studies it could also be fruitful to incorporate focused and unfocused note-taking conditions. Comparing both types of testing  to both types of note-taking would provide deeper insight into the degree to which specific testing is desirable because it focuses learners on the relevant content items or engages learners in retrieval practice and elaboration.The raw data supporting the conclusions of this manuscript will be made available by the authors, without undue reservation, to any qualified researcher.SH, AG, KB, SF, and JR designed the experiments. SH organized the data collection and the data base. SH and JR performed the statistical analyses. SH, AG, SF, and JR interpreted the results. SH and JR wrote the first draft of the manuscript. AG, KB, and SF revised sections of the manuscript. All authors read 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.


Retrotransposable elements (RTEs) have actively multiplied over the past 80 million years of primate evolution, and as a consequence, such elements collectively occupy ∼ 40% of the human genome. As RTE activity can have detrimental effects on the human genome and transcriptome, silencing mechanisms have evolved to restrict retrotransposition. The brain is the only known somatic tissue where RTEs are de-repressed throughout the life of a healthy human and each neuron in specific brain regions accumulates up to ∼13.7 new somatic L1 insertions (and perhaps more). However, even higher levels of somatic RTE expression and retrotransposition have been found in a number of human neurological disorders. This review is focused on how RTE expression and retrotransposition in neuronal tissues might contribute to the initiation and progression of these disorders. These disorders are discussed in three broad and sometimes overlapping categories: 1) disorders such as Rett syndrome, Aicardi-Goutières syndrome, and ataxia–telangiectasia, where expression/retrotransposition is increased due to mutations in genes that play a role in regulating RTEs in healthy cells, 2) disorders such as autism spectrum disorder, schizophrenia, and substance abuse disorders, which are thought to be caused by a combination of genetic and environmental stress factors, and 3) disorders associated with age, such as frontotemporal lobar degeneration (FTLD), amyotrophic lateral sclerosis (ALS), and normal aging, where there is a time-dependent accumulation of neurological degeneration, RTE copy number, and phenotypes. Research has revealed increased levels of RTE activity in many neurological disorders, but in most cases, a clear causal link between RTE activity and these disorders has not been well established. At the same time, even if increased RTE activity is a passenger and not a driver of disease, a detrimental effect is more likely than a beneficial one. Thus, a better understanding of the role of RTEs in neuronal tissues likely is an important part of understanding, preventing, and treating these disorders. Approximately 45% of the human genome is derived from insertions of DNA sequences known as transposable elements (TEs) . There aAlu (∼11%) (Alu (SVA) (∼0.2%) (There are two types of non-LTR retrotransposons in humans: Long interspersed elements (LINEs), and short interspersed elements (SINEs) . Long inu (∼11%)  and SINE (∼0.2%) . They ar (∼0.2%) .Alus  that are necessary for retrotransposition Figure 1Alus  and SVAsAlus  are mobiAlus .de novo L1 insertions were initially thought to occur only in the germline, it is now clear that somatic L1 retrotransposition occurs in the healthy human brain in neural progenitor cells (NPCs) and during neurogenesis .Alu and SVA germline insertions were identified, along with ∼22,000 somatic insertions . They selected 33 candidates for validation experiments  and were able to verify 28/33 (85%) by PCR. They found that in contrast to germline insertions, somatic insertions were disproportionately located in genes associated with neurogenesis and synaptic function, suggesting that retrotransposition may alter the genetic circuitry of neurobiological processes  is a rare, usually sporadic genetic postnatal neurological disorder that shows symptoms at 6–18 months of age and leads to severe impairments, including slow development, intellectual disability, seizures, and autistic behaviors. It is caused by mutation of the X-linked gene methyl CpG binding protein 2 (MECP2), which leads to abnormal epigenetic regulation . MeCP2p −/− mice and RTT NPCs engineered to carry an L1–EGFP construct, −/− mice compared to the wildtype mice. The brains of RTT patients also were analyzed for L1 ORF2 copy number by qPCR and had signiﬁcantly higher L1 ORF2 copy numbers compared to age and gender-matched controls and heart tissues from the same patients. Using chromatin immunoprecipitation (ChIP) and qPCR, RTT was the first neurological disorder where disease-related genetic mutations were found to result in increased levels of L1 retrotransposition in neurons and neuronal precursors . MeCP2p Alu Y retrotransposition were observed in the NPCs from diseased brains, with lower levels of L1Hs and Alu Y in NPCs from non-diseased brains and non-brain tissues. Members of the L1Hs and Alu Y families are the youngest and most active RTEs in humans. The overall number of L1Hs insertions in these neurological diseases was 8.5 fold higher than the normal brain controls, and somatic L1Hs insertions in particular were 14.7 fold higher than controls . AGS is a rare inherited encephalopathy resulting in severe mental and physical handicaps. About 20% of the cases are an early-onset form where infants are born with neurological and liver abnormalities including enlargement of the liver and spleen and elevated liver enzymes. Symptoms closely resemble a congenital viral infection of the brain, including elevated levels of type I interferon (IFN) in the serum and cerebrospinal fluid . MutatioBut how is L1 linked to AGS? The finding that retroelements are TREX1 substrates was validated using two genetically marked retroelements: 1) a human L1 element and 2) an LTR-containing murine intracisternal type A particle (IAP), each containing a neomycin retrotransposition indicator cassette. In HeLa cells, transfection with the wild-type IAP plasmid resulted in abundant neomycin-resistant colonies indicating the production of retrotransposition events. However, when an expression vector for TREX1 was co-transfected, the retrotransposition efﬁciency was reduced to less than 40% of the controls. In similar experiments with L1, the co-transfected TREX1 reduced L1 retrotransposition by over 80% .An increase in extrachromosomal L1 retrotransposon DNA also was observed in TREX1-deficient neural cells and human pluripotent stem cells . Two celSequencing of extrachromosomal DNA from one of the TREX1-deﬁcient NPC lines, its isogenic control, and an RTi-treated TREX1-deﬁcient line, showed that extrachromosomal L1 DNA was abundant when TREX1 was inactive, and in particular, there was 70% more L1Hs in the TREX1- deﬁcient line compared with the control. In contrast, the RTi-treated, TREX1-deficient cell line had nearly normal levels of L1 and L1Hs extrachromosomal DNAs. This is consistent with the extrachromosomal DNAs being generated by the L1 RT (encoded by L1 ORF2p). Interestingly, later work by The most common cause of AGS, found in over 50% of AGS patients, is biallelic mutation of one of the three RNase H2 subunits  . Such muAlu and SVA retrotransposition also was observed . AT is a rare inherited neurodegenerative disorder that is caused by mutations in the ataxia telangiectasia mutated (ATM) gene, which encodes a serine/threonine kinase that is involved in DNA damage sensing and repair signaling. AT mainly affects the immune and nervous systems and is characterized by progressive difficulty with coordinating movements beginning in early childhood .In a study of AT by in vitro and yet most genomic L1 insertions are 5′ truncated, Additional supporting evidence of increased L1 activity in AT patients was discussed earlier where As with the other two examples where disease-related genetic mutations led to increased frequencies of neuronal L1 expression and retrotransposition , L1 activity likely is not the underlying cause of AT. However, it is possible that it contributes to some of the symptoms or to the progression of these diseases. In addition to the potentially deleterious effects that might be caused by somatic retrotransposon insertions (particularly those that disrupt genes), the endonuclease of L1 ORF2p has the potential to cause DNA damage even in the absence of a successful retrotransposition event . LikewisUnlike RTT, AGS, and AT, a single gene  has not been found to cause autism spectrum disorder (ASD). However, estimates of the heritability of ASD from twin studies are as high as 93% indicating that genetic factors contribute to the disease . The CenStudies of genetic changes associated with ASD have mainly been focused on germline changes, however recent studies have shown increased levels of somatic L1 activity in postmortem brains of ASD patients . As discIn postmortem brain samples from 13 individuals with ASD, Because According to the National Institute of Mental Health, schizophrenia is estimated to affect between 0.25 and 0.64%  of indivImmune activation models simulating both viral infection and inflammation also have been used to investigate possible links between perinatal environmental risk factors for schizophrenia and L1 activity. Viral infection was simulated by injecting pregnant mice with polyinosinic:polycytidylic acid [poly(I:C)], which mimics viral double-stranded RNA. Inflammation was simulated by chronic injection of epidermal growth factor (EGF) into neonatal macaques. In both the mouse and macaque models, an increase in brain L1 copy number was observed in response to these two perturbations, indicating that the L1 content of the brain likely is influenced by early environmental factors .One of the environmental risk factors for schizophrenia is early childhood trauma, and the association of this risk factor with L1 expression was demonstrated by The first study to find an impact of substance abuse on L1 expression was by de novo mutations; a small fraction of the insertions were possibly somatic L1 insertions that were associated with cocaine addiction. No changes in L1 transcription or retrotransposition were observed in bulk medial prefrontal cortex tissues from individuals with cocaine addiction. However, the novel L1s in these individuals were enriched in genes that previously have been associated with cocaine addiction. This supported the authors’ hypothesis that inherited or somatic L1s in neuronal genomes may predispose an individual to cocaine addiction  and cognitive decline are typically associated with aging, or from a cumulative effect of perturbations or alterations in cellular mechanisms over time. FTLD is a process of shrinkage of specific areas of the brain that regulate behavior, personality, and language. Up to 40% of patients with this disease have a family history of FLTD. FTLD is the second most common cause of dementia in patients younger than 65 years of age and it accounts for another 25% of dementia cases in patients older than 65 years . The accBased on data from a meta-analysis of publicly available TDP-43 data, protein:RNA target binding datasets, and mRNA expression datasets, in vivo, in a Drosophila model of FTLD where human TDP-43 (hTDP-43) was overexpressed from a hTDP-43 transgene . An LTR retrotransposon, i.e. gypsy, was selected to test the impact of hTDP-43p on induced retrotransposon expression. In glia, gypsy was induced by hTDP-43p expression and the resulting toxicity could be partially rescued through the use of an RTi, suggesting that gypsy caused the degenerative phenotypes in Drosophila. Further, the shortened lifespan in the hTDP-43p-expressing flies was fully rescued by blocking the expression of Loki expression, showing that hTDP-43p toxicity is predominately caused by Loki/Chk2 activation of DNA damage-mediated programmed cell death  and ALS  a major cluster of samples with oxidative and proteotoxic stress (61% of patients), (2) a second cluster with glial activation and inflammation (19% of patients) and (3) a third cluster with retrotransposition reactivation (20% of patients) . Since oIn a separate study Liu et al. showed that TDP-43 normally inhibits L1 retrotransposition . In partAlu methylation but not L1 methylation  to map genome-wide chromatin conformations in human fibroblasts in cell culture, chromatin was found to undergo extensive changes during replicative senescence with an overall closing of chromatin in euchromatic gene-rich regions and a relaxation of heterochromatin in gene poor and pericentromeric regions , and an LTR retrotransposon (gypsy), were found to be highly active in the Drosophila brain during normal aging (Drosophila Argonaute 2 (dAgo2), there was increased R2 and gypsy expression in the brain of young Drosophila and the increased expression was correlated with an accelerated age-dependent memory impairment and shortened lifespan. In addition, there was a significant delay in mortality and age-dependent memory impairment when an RNAi transgene was used to target Loki/Chk2. Based on these results, the authors suggested that transposon activation may be a contributing factor in age-dependent loss of neuronal function (In al aging . By mutafunction . RecentlThese studies have shown that a relaxing of RTE silencing occurs with age, and the aberrant expression and increased retrotransposition activity of RTEs may result in DNA damage. In total, these studies show that retrotransposons are progressively reanimated as organisms age, but the extent to which this is a symptom of or a cause of cognitive decline is unclear. Additional work is needed to determine whether the increased RTE activity that occurs over time impacts aging or is instead a symptom of aging.RTEs have actively proliferated over the past 80 million years of primate evolution. As an evolutionary response to their potentially detrimental effects on the human genome and transcriptome, silencing mechanisms have evolved to restrict RTE expression and retrotransposition. The brain is the only known somatic tissue where RTEs are de-repressed throughout the life of a healthy human. In the diseases outlined above, RTEs are improperly regulated and often are overexpressed beyond normal levels in the diseased state. Whether this is a marker of the disease or a driver is, in most cases, poorly understood. However, increased mutagenesis by mobile elements likely is detrimental, and may very well contribute to disease progression. In addition to deleterious effects from somatic retrotransposon insertions, which can disrupt genes or otherwise dysregulate gene expression, the by-products of the retrotransposition process also may have detrimental effects. For example, the ORF2p product of L1 can cause mutations and instability through its endonuclease activity and L1 expression may result in a buildup of RNA and/or DNA in the cytosol trigging an immune response, inflammation and neuron degeneration. While research shows increased levels of RTE activity in many neurological disorders, additional work is needed to establish the extent to which RTE activity impacts these disorders. Thus, a better understanding of the role of RTEs in neuronal tissues likely is an important part of understanding, preventing, and treating these disorders.DT and SD reviewed the literature and prepared the manuscript. DT designed the figures and This work was supported by grant R01HG002898 from the National Human Genome Research Institute, National Institutes of Health.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.


Electromagnetically induced transparency (EIT) arises from the coherent coupling and interference between a superradiant (bright) mode in one resonator and a subradiant (dark) mode in an adjacent resonator. Generally, the two adjacent resonators are structurally or spatially asymmetric. Here, by numerical simulation, we demonstrate that tunable EIT can be induced by graphene ribbon pairs without structurally or spatially asymmetry. The mechanism originates from the fact that the resonate frequencies of the bright mode and the dark mode supported by the symmetrical graphene ribbon pairs can be respectively tuned by electrical doping levels, and when they are tuned to be equal the graphene plasmon coupling and interference occurs. The EIT in symmetrical nanostructure which avoids deliberately breaking the element symmetry in shape as well as in size facilitates the design and fabrication of the structure. In addition, the work regarding to EIT in the structurally symmetric could provide a fresh contribution to a more comprehensive physical understanding of Fano resonance. This concept was later extended to classical optical systems and induced by interference between a bright mode in one optical resonator and a dark mode in an adjacent optical resonator4. With the characteristic of high quality factor, steep dispersion and near field-enhanced, EIT has many applications ranging from signal processing5, sensors8, lasing10, nonlinear11 and slow-light devices13. Recently, metal nano-plasmonic structures have attracted great attention due to their ability of confining light to sub-wavelength dimensions and opening up the possibilities to construct optical devices with various scales and shapes while maintaining consistent optical properties that do not depend on the dimensions of the device. Many metallic plasmonic structures have been designed to achieve the EIT20. A direct effective way for inducing EIT is destroying the symmetry in metallic structures. For example, by deliberately breaking the element symmetry in shape as well as in size, the concentric double rings23, ring-disk composite24, asymmetric split-ring pairs25, and mismatched nanoparticle pairs26 can induce EIT. Although most of these symmetry breaking nanostructures exhibit obvious EIT, they are complex in structure, difficult to design and sensitive to preparation accuracy. The EIT with simple structure and high preparation tolerance such as general symmetrical structures are desirable. However, owing to the difficulty in generation of spectrally overlapping plasmonic resonances with very different radiance in highly symmetric nanostructures, there’re few reports about the EIT based on highly symmetric plasmonic nanostructures. In addition, metallic plasmonic nanostructures are hard to tune due to the limitation of material, which make it usually necessary to refabricate the physic structure in order to achieve the tunability. But, in many practical applications, it’s hard to change the physical structure after manufacturing. So, it is meaningful and challenging to get tunable EIT without structurally or spatially asymmetry.Electromagnetically induced transparency (EIT) is a concept originally observed in atomic physics where the coherent coupling of a broad and a narrow resonance leads to quantum interference27, a novel semi-metal material, has attracted tremendous attention due to its unique optical and electrical properties, such as low loss, ability to support plasmonic resonance in the terahertz and mid-far infrared bands30, strong field localization31 and nonlinear optical effects32. So far, graphene has widely applications on photonics devices ranging from ultrafast pulsed lasers37, graphene polarizer39 and light emitting devices40. More importantly, with electrical gating42 and chemical doping43, graphene shows the ability to tune plasmonic resonance. Some researchers have proposed and studied various graphene structures to provide tunable plasmonic resonance, for instance, graphene film44, graphene nanorings46, graphene patches47, graphene photonic crystal structures48. Specially, nanorings and graphene patches have been used to design the tunable asymmetric EIT structure47.Fortunately, graphene28, and can easily realize different Fermi energy on different ribbons at the same time. We elucidate the physical mechanism firstly. Then, we present numerical simulations and results. Finally, we investigate the effects of some important structural parameters. It is worth mentioning that our structure can not only utilize graphene as the plasmonic platform to excite EIT, but also use other two dimensional materials like black phosphorus (BP)51 and antimonene52.In this work, we demonstrate that tunable EIT can be induced by graphene ribbon pairs without structurally or spatially asymmetry. Here, the reason for using graphene ribbon pairs is that the graphene ribbon is the simplest form of sub-wavelength graphene plasmon structure and has been widely fabricated by different researcherst of the dielectric substrate is set to be semi-infinite.The proposed structure is presented in Fig. 54. The corresponding N-order graphene plasmon resonance of a separate ribbon occurs atN is a positive integer determining the order of a resonance mode, Re55ħ is the reduced Planck’s constant, for the graphene ribbons with one side exposed to the surrounding medium with dielectric constant c is the light speed of the vacuum.The doped graphene behaves as an essentially 2D electronic system. So, the electromagnetic waves coupled to collective charge excitation (plasmons) can be excited in the doped graphene. The plasmons become localized in the patterned graphene because of the confinement of spatial dimension. The localized plasmons correspond to collective charge oscillation modes of various orders of standing waves. Therefore, the doped graphene ribbons providing confinement along one spatial dimension can be predicted to produce sharp plasmon resonances across the width of ribbonsFrom Eq. , we find5657.To verify the theoretical prediction, we next conduct full-wave numerical simulations employing frequency domain solver in CST Microwave Studio. In our simulation, the dielectric substrate is set to be a normal non-dispersive material with relative permittivity First, we consider the case of arrays of single graphene ribbon where entclass1pt{minimaentclass1pt{minimaThen, we investigated the situation of arrays of symmetric graphene ribbon pairs with two ribbons doped by different Fermi energy bon Fig.  and arrabon Fig. . As for ets Fig. . On the ion Fig. . However60The EIT can be regarded as an analogue of Fano-type resonance. To further confirm the EIT in the structure, we fit our simulation results with the general form of the Fano-type resonance, which can be written asmentclass2pt{minimIn order to investigate the characteristics of the EIT of this structure, we sweep a few parameters, including the Fermi energy 24. As shown in Fig. We first calculated the transmission spectra as functions of frequency when the Fermi energy From the results above and Eq. , one can61, a smaller duty ratio of structure leads to a smaller equivalent refractive index, and result in a larger resonance frequency of the dark mode. In order to discuss the influence of Figure with Eq. . The surFrom Eq. , one can63. The detailed process for fabrication is shown in Fig. 28. Then, two metal electrodes are deposited and respectively contact on one ribbon of graphene ribbon pairs along the Finally, we discuss how the Fermi energy In conclusion, we demonstrate that EIT can be induced by arrays of graphene ribbon pairs without structurally or spatially asymmetry. By changing two external gate voltages, the Fermi energy of two graphene ribbons in a ribbon pair can be physically controlled, respectively, so the EIT can be tuned without refabricating the physic structure. At the same time, in contrary to EIT in a structurally or spatially asymmetry structure where structurally asymmetric is introduced by deliberately breaking the element symmetry in shape as well as in size, EIT in the symmetry structure facilitates the design and fabrication of the structure. In addition, since the EIT is a Fano-type response, the work regarding to EIT in the structurally symmetric could provide a fresh contribution to a more comprehensive physical understanding of Fano resonance.


The enhanced TEC performance is attributed to improvements in the electrical and thermal conductivities, as well as the adhesion between the CNT-Gr hybrid and the substrate. Meanwhile, the relative conversion efficiency of the TECs can reach 1.35%. The investigation suggests that the growth of CNT-Gr hybrid electrodes by the EPD technique may offer a promising approach for practical applications of the carbon nanomaterial-based TEC electrodes.Carbon nanotube-Graphene (CNT-Gr) hybrids were prepared on stainless steel substrates by the electrophoretic deposition (EPD) to make the thermo-electrochemical cell (TEC) electrodes. The as-obtained TEC electrodes were investigated by the SEM, XRD, Raman spectroscopy, tensile, and surface resistance tests. These hybrid electrodes exhibited significant improved TEC performances compared to the pristine CNT electrode. In addition, these hybrid electrodes could be optimized by tuning the contents of the graphene in the hybrids, and the CNT-Gr-0.1 hybrid electrode showed the best TEC performance with the current density of 62.8 A·m The thermo-electrochemical cell (TECs) also called as thermo cell or thermogalvanic cell ,2,3, conoc) between the two working electrodes are two important parameters for the TECs. The relation between I and Voc can be obtained by the equation: I = Voc/R, where Voc represents the open-circuit potential, which is mainly determined by the temperature difference [The current (I) and the voltage  were added into ethanol solution and sonicated for about 2 h to form the stable suspension. The stainless-steel substrate and the counter electrode (the SS foil) were immersed into the suspension with a distance of 4 cm and the constant voltage of 120 V. Finally, the CNT-Graphene hybrid was annealed in the furnace at 700 °C with the duration of 1 h. The TEC performances of the CNT-Graphene hybrids were optimized by tuning the concentration of Graphene. The as-prepared hybrids are denominated as CNT-Graphene-x, where x is the concentration of the Graphene from 0 to 0.4 g L−1. The main synthesis processes of the CNT-Graphene hybrid are illustrated in The CNT-Graphene hybrids were synthesized on the stainless steel (SS) substrate by the EPD method. Firstly, the stainless-steel substrates and the multiwalled carbon nanotubes  were ultrasonically cleaned and carboxylic pretreated, respectively, as described in our previous reports ,10. SecoThe morphologies of the pristine CNTs and the CNT-based hybrids were observed using scanning electron microscopy . The species of all samples were identified by X-ray diffraction  and Raman spectroscopy (Renishaw Invia Raman Microscope with the laser wavelength of 633 nm). To investigate the adhesion between the pristine CNT or the CNT-based hybrid and the SS substrates, tensile tests were performed by an Instron 3343 equipment [−1, Aladdin) aqueous solution, the temperatures of two test sides were controlled by the heating tape and ice water, respectively, and measured by OMEGA thermocouple probes. The TEC parameters, such as short-circuit current and the open-circuit potential, were acquired by the KEITHLEY 2440 multimeter .The CNT and the CNT-based hybrid electrodes were tested by the Cup-Shaped TEC equipment and the Cup-Shaped device of TEC and characterization of TEC performance were described in other reports and our previous investigations ,5,11. Br−1 , G band (1585 cm−1), and G’ (2706 cm−1), respectively [G/ID ratio, the better the conductivity of the sample [G/ID ratio of the CNT-Graphene-0.1 hybrid suggests that the construction of CNT-Graphene hybrid would enhance the electrode conductivity.XRD and Raman characterizations were applied to check the compositions of the products, as shown in ectively ,24. The ectively . In compe sample ,25,26. C−1, demonstrating significant reductions of surface resistances with graphene doping. The CNT-Graphene-0.1 hybrid electrode exhibited the lowest resistance, attributed mainly to two reasons. Firstly, comparing with other grapheme doped electrodes, the CNT-Graphene-0.1 electrode presented the highest adhesion value  increased with the temperature differences  under a temperature difference of 50 °C.The Cup-Shaped device was used to investigate the TEC performances of the pristine CNT and CNT-Graphene hybrid electrodes ,11. As sctrolyte ,5,11. Foferences b. Compartivities ,28,29 anext) according to the relation: U = Voc–IR, where Voc is the open-circuit potential, which is mainly determined by the temperature difference [oc can be regarded as a constant, thus the relationship between U and I is approximately linear. During the continuous operation for the TEC, the built up concentration gradient, i.e., the mass transport overpotential, could be easily formed at the cold side [oc–IR) I = Voc I–I2R, thus a quadratic relationship can be observed in the P–I curve  with the test time, and the plots of the specific output power (P) of the TECs for the pristine CNT and CNT-Graphene-0.1 hybrid electrodes were shown in fference , and R rfference ,14,30. Fold side ,11. The old side ,27,28. T–I curve d. As shojunction .η) can be obtained using the following formula [scI and ocV are the short-circuit current and the open-circuit potential, respectively. A is the area of the test electrode, k is the thermal conductivity of the potassium ferri/ferrocyanide, ∆T and d are the temperature difference and the distance of the test electrodes, respectively. The relative energy conversion efficiency (ηr) can be achieved by the equation: ηr = η/(∆T/Th) [h is the temperature of the hot side. Based on the above equations, ηr of the pristine CNT electrode and CNT-Graphene-0.1 hybrid electrode reached 0.95% and 1.35%, respectively, at the temperature difference of 50 °C. Comparing with the CNT and CNT-based hybrid electrodes, the TEC performance of CNT-graphene-0.1 was moderate  ,5,11, whrate See . Future sistance ,33.−2 at a distance of 3 cm, 39.88% higher than that of 7 cm at the temperature different of 30 °C. −2 at the distance of 3 cm, associated mainly to the drops of the internal resistance and the mass transport overpotentials [The temperature difference versus current density, the discharge curves, the fitting results of the internal resistance, and the plots of the specific output power of the TECs for the CNT-Graphene-0.1 hybrid electrode with different separation distances are shown in tentials ,14. Howe−2 and the power density of 1.15 W·m−2 at a temperature different of 50 °C, 30.4% higher than the pristine CNT electrode. Meanwhile, the relative conversion efficiency of 1.35% could be reached. These results suggest that the production of CNT-Graphene hybrid electrodes by EPD technique may offer a promising approach for developing CNT-based TEC electrode materials.In this paper, CNT-Graphene hybrids were synthesized on stainless steel substrates using the electrophoretic deposition (EPD) technique to make the TEC electrodes. Such hybrid electrodes show significantly improved TEC performances in comparison with the pristine CNT electrode, due to better electrical and thermal conductivities and stronger adhesion between the CNT-Graphene hybrid electrode and the substrate. By tuning the concentrations of the graphene in the suspension, the hybrid electrodes could be optimized with the maximum current density of 62.8 A·m


An efficient electron field emitter based on a monolayer graphene coated well aligned Mo tip array has been designed, fabricated, and evaluated. The advantages of this hybrid nanostructure film morphology are explored and discussed. Efficient and stable field emissions with low turn-on fields have been observed with the new devices. It is further found that the combination of graphene and Mo tip array leads to significant improvements in efficiency for the nanoscale heterostructure emitters. Electron field emission is a quantum tunneling phenomenon whereby electrons are emitted from a solid surface which is affected by a strong electric field . Due to The materials used for cold cathode emitter must exhibit field enhancing effects internally or externally . NanostrSignificant enhancement of electron field emission capability can only be achievable from the use of sharp edged graphene when the electric field is applied along the sheet due to the local field enhancement at the edge . PracticIn this paper, through device parameter optimizations, we investigated the field electron emission characteristics of graphene-coated Mo tip array that can be used to fabricate an emerging class of highly efficient nanostructured field emitters. Compared with pristine Mo tip array, and graphene-coated flat Mo chip, the proposed graphene-coated Mo tip array was evidently observed with improved emission efficiency and a reduced turn-on field. It is believed that the improved performance is due to the reduction of effective field emission tunneling barrier, which is generated by graphene-metal charge transfer interactions. I) as a function of the applied electric field (E), which can be expressed as . T. T3F plaGermany) e. ResiduGermany) f. PMMA/gGermany) g by stanGermany) h.D/h ≈ 1.6, was implemented with good uniformity and a high aspect ratio ≈ 1.8, shown in D remaining broadly the same, thus enlarge the D/newh. From the simulation above, with the enlarged D/newh, the emission current is improved significantly. The planar bulk Mo chip before and after being functionalized with graphene was characterized by scanning electron microscopy, as shown in A Mo tip array, of 10 μm pitch with DI/GI ratio of the graphene-coated Mo tips and SEM micrographs indicates an enhancement in defect density in the graphene-coated Mo tips, likely induced by the transfer process and high basal plane stresses induced at the Mo tip apexes. Such defects likely provide additional new emission sites as the variation in field is much larger at defects, edges, and ripples at the atomic scale, which leads to larger local electric fields. The surface roughness of the graphene sheets on the Mo tips, as shown in Raman shifts of the graphene-coated Mo tips and graphene-coated flat Mo chip are shown in −7 mbar in a custom-built, automated field emission system. All measurements were performed at a cathode-to-anode distance of 1 mm. An ITO/glass anode was positioned adjacent to the emitting surface and the emission area of all the three samples was 12.5 mm2. The emission characteristics were measured from 0–5 kV, at 50 V increments, with spectra consisting of both up and down sweeps. The emission current was averaged (n = 3) at each bias. The samples were measured at a base pressure of <10Three different FE cathodes were measured and compared: pristine Mo tip array, graphene-functionalized Mo tip array, and graphene-coated flat Mo chip. Typical I-V characteristics are shown in −4 A, around an order of magnitude greater than the maximum emission current of the unfunctionalized Mo tips (5.70 × 10−6 A). There was little measurable emission current above the SMU noise floor from the graphene sheets on flat Mo chip, even under the maximum applied electric field of 5 × 106 V/m, as shown in The maximum emission current of graphene-coated Mo tip array was 1.27 × 10D is approximately three times the height h. For the Mo tip array, D/h ≈ 1.6, suggesting that the cones are overly closely packed for the optimal emission. Upon coating Mo tip with the CVD graphene, we noted the occurrence of new conical surface features, where the dimensions of these new cones are comparable to those of the original Mo tips, though the height of the new cones are somewhat shorter, with D remaining broadly the same, thus enlarging D/newh and, thus, adjusting the emission current density.Controlling the tip areal density was also important in determining the field emission performance, as electron screening from neighboring tips must be minimized. Our simulations suggest that the emission current density reaches a maximum when the distance Additionally, after coating Mo tip array with graphene, field emission occurred at considerably lower turn-on fields. We attribute this to the possible formation of a triple junction between Mo, graphene, and vacuum  coupled In this paper, an innovative design for using a hybrid graphene-coated Mo tip array for efficient field emission is demonstrated, designed and fabricated. Both simulation and testing results demonstrate the new present methodology leads to lowering of the turn on field and enhanced maximum emission currents. The maximum emission current of the graphene-functionalized Mo tip array is 22 times larger than the pristine Mo tip array. The feasibility of depositing wrinkled graphene sheets in large scale will allow further investigation of the new devices, as well as exploiting their unique 2D structure for many potential applications. This work could pave the way for the design and applications of future electron emission 2D heterostructure nano devices.


In contrast, with an increase in temperature, the resistance of the BN/graphene/BN heterostructure without a microcavity decreased with a temperature coefficient of −0.16%°C−1. The linearity of the resistance change rate (ΔR/R)–temperature curve of the BN/graphene/BN heterostructure without a microcavity was better than that of the BN/graphene/BN heterostructure with a microcavity. These results indicate that the influence of temperature on the pressure sensors based on BN/graphene/BN heterostructures should be considered, especially for devices with pressure microcavities. BN/graphene/BN heterostructures without microcavities can be used as high-performance temperature sensors.Temperature is a significant factor in the application of graphene-based pressure sensors. The influence of temperature on graphene pressure sensors is twofold: an increase in temperature causes the substrates of graphene pressure sensors to thermally expand, and thus, the graphene membrane is stretched, leading to an increase in the device resistance; an increase in temperature also causes a change in the graphene electrophonon coupling, resulting in a decrease in device resistance. To investigate which effect dominates the influence of temperature on the pressure sensor based on the graphene–boron nitride (BN) heterostructure proposed in our previous work, the temperature characteristics of two BN/graphene/BN heterostructures with and without a microcavity beneath them were analyzed in the temperature range 30–150 °C. Experimental results showed that the resistance of the BN/graphene/BN heterostructure with a microcavity increased with the increase in temperature, and the temperature coefficient was up to 0.25%°C It has a lower thermal conductivity [1.9 W/(m·K)] and a higher compressive strength in the direction parallel to the c-axis [Graphene and its composites have become an important frontier of international research owing to their excellent electrical, mechanical, and thermal properties, and their two-dimensional structures ,2,3,4,5.–50 kPa) . Tao LQ –20 kPa) . Owing t–20 kPa) . Boron n–20 kPa) ,16. BN ie c-axis ,18. As te c-axis . In addiIn practical applications, graphene and its composites are easily subjected to thermal stress and thermal deformation owing to large changes in their ambient temperature; hence, temperature is often a non-negligible factor for graphene pressure sensors, and has a considerable impact on BN/graphene/BN heterojunctions. There are few studies on the temperature characteristics of a sensor based on BN/graphene/BN heterojunction. This study aims to analyze this problem. For the temperature characteristics of monolayer graphene, Corey et al. prepared graphene/polyvinylidene difluoride thin films and studied their electrothermal properties. The temperature coefficient of resistance changed slightly with the mass fraction, indicating that the electron transport mechanism of various mass fractions was the same . Sun andElectrophonon coupling is an important factor affecting the temperature sensitivity of graphene, and it is crucial to understand the metal–semiconductor properties of graphene . The incWhen lattice vibration is considered, the atom (ion) deviates from the equilibrium position and causes a change in potential energy. The band electrons will be affected by the additional potential field generated by the lattice displacement, which is the interaction between the electrons and the lattice vibration. Electron–phonon interaction refers to the interaction between electrons and lattice vibrations. Positively charged atoms in a solid form a lattice at their static equilibrium, and the periodic field of the lattice causes the energy spectrum of the electron to exist as an energy band. In the periodic field, electrons have definite energy and the term “quasi momentum” k, where k is the wave vector, k = h/2π, and h is the Planck constant. It shows that the band electrons behave like free electrons. The action of the periodic field can be attributed to the effective mass of the electron. As the atoms are implicated in each other, the vibrations of the atoms form various lattice waves, namely, simple harmonic modes  with different frequencies, wave vectors, and polarizations. The energy quantum of each normal mode is a phonon. Hence, the interaction between an electron and phonon represents the interaction between an electron and lattice vibration. It can be concluded from the Kubo–Greenwood formula that the conductivity of graphene decreases with an increase in temperature, and the effect of temperature on graphene alone will reduce the resistance of graphene.Previous studies have shown that the influence of temperature on the conductivity of graphene involves a node that divides the entire temperature range into two parts . It has Graphene and boron nitride are mainly prepared on a copper base via chemical vapor deposition (CVD); subsequently, they are transferred to the target substrate through a wet transfer method and heterogeneous combination is completed. The thickness of the copper base is approximately 20 µm, the thickness of boron nitride is 13 nm, and boron nitride coverage is more than 90%. Boron nitride is transferred to the target via wet transfer. First, its surface is coated with poly (methyl methacrylate) (PMMA) to protect it from pollution and destruction. It is subsequently soaked in a ferric chloride solution for etching the copper base with a target base PMMA/BN film and cleaned. Finally, the transfer is completed and PMMA is removed. The graphene transfer process is also similar. Elastic thin films were prepared on a silicon/silicon dioxide substrate via inductively coupled plasma etching, and subsequently, graphene thin films were assembled on BN thin films via a fixed-point transfer technique. Subsequently, a BN/graphene/BN heterojunction was assembled and synthesized. Finally, photolithography, sputtering, and other MEMS processes were used to prepare the interconnected electrode and package structure.2 wafer, and the graphene upper and lower layers were coated with metal to form an electrode contact. For the graphene pressure sensor with a cavity, first, the plasma-enhanced CVD (PECVD) technique was used to deposit SiO2 of thickness 1–2 µm on the bare Si wafer to form the device insulation layer. AZ5214 photoresist was used as the mask, and SF6, CHF3, and O2 were used to etch the cavity structure (depth of 500–700 nm) on SiO2. The bottom electrode of the device was prepared by magnetron sputtering Ti (10 nm)/Pt (50 nm) to form the bottom electrode contact of the sensitive unit. After the transfer, photolithography, and etching of the underlying h-BN and graphene, the top electrode of Au (50 nm) was prepared via electron beam evaporation. Finally, the transfer and graphics of the top h-BN were completed. Thus, the cavity-type graphene temperature sensor was fabricated. For the non-cavity-type graphene pressure sensor, the process of cavity etching is not required. The other processes are the same as those used for the cavity-type graphene temperature sensor.The sensing unit of each graphene temperature sensor consists of a single layer of CVD graphene sandwiched between two layers of CVD h-BN. Using the wet transfer technique supported by large-area PMMA, two-dimensional heterogeneous structures were prepared on a Si/SiO2 as the insulating layer, graphene wrapped by two layers of BN, and a Au electrode that forms ohmic contact with graphene. The processing technology of the cavity-type graphene sensor is described in [2. Scanning electron microscopy (SEM) diagrams of the two graphene sensors are shown in The processing technology of the non-cavity-type graphene sensor is shown in ribed in . In cont2, and it appears near 1580 cm−1. This peak can effectively reflect the number of layers of graphene, but it is easily affected by stress. The D peak is generally considered to be the disorderly vibration peak of graphene. The specific location of the peak is related to the laser wavelength. It is caused by the lattice vibration leaving the center of the Brillouin region and is used to characterize the structural defects or edges in graphene samples. The G’ peak, also known as the 2D peak, is the second-order Raman peak of double phonon resonance, which is used to characterize the interlayer stacking mode of carbon atoms in graphene samples. Its peak frequency is also affected by the laser wavelength. Specifically, monolayer graphene has two typical Raman characteristic peaks: the G peak near 1582 cm−1 and the 2D peak near 2700 cm−1. Furthermore, for graphene samples containing defects and with a certain degree of disorder, the D peak at approximately 1350 cm−1 is observed. The Raman spectrum characteristics of graphene materials can be determined by combining the appearance, peak strength, peak shape, peak position, and the relationship between different Raman characteristic peaks. Raman characterization showed that BN protection had no effect on the properties of graphene. We performed Raman test on the monolayer graphene and the BN/graphene/BN heterostructure, as shown in In terms of the transfer mass of graphene, it is crucial to determine its number of layers and quantify its disorder. Laser microscopic Raman spectroscopy is a standard and ideal analytical tool to characterize the above two properties. By obtaining the Raman spectrum of graphene, we can evaluate the structure and properties of graphene, such as the number of layers, stacking mode, defect number, edge structure, tension, and doping state. In addition, Raman spectroscopy plays an important role in clarifying the electronic phonon behavior of graphene. The Raman spectrum of graphene is composed of several peaks, mainly the G, D, and G’ peaks. The G peak is the main characteristic peak of graphene, which is caused by in-plane vibration of the carbon atom spI)–voltage (V) characteristic using a four-probe tester. Six samples of each structure were measured. The resistances of the BN/graphene/BN pressure sensor with a microcavity are approximately 1050 Ωsq−1, 1092 Ωsq−1, 1065 Ωsq−1, 1105 Ωsq−1, 1083 Ωsq−1, 1170 Ωsq−1 and that of the BN/graphene/BN pressure sensor without a microcavity are approximately 1311 Ωsq−1, 1320 Ωsq−1, 1330 Ωsq−1, 1350 Ωsq−1, 1380 Ωsq−1, 1400 Ωsq−1. The I–V curve of the BN/graphene/BN pressure sensors indicates that graphene retains its high quality after the top BN protective layer is coated. The electrical characterization of one of the selected samples is shown in We measured the current (R0, R, and △T are the resistance at the initial temperature T0, the resistance at temperature T, and the D-value between the temperature T and the initial temperature T0.To test the temperature characteristic of the two pressure sensors, the resistance change was measured using a high-temperature drying oven with a change in temperature from 30 to 150 °C. Six samples of each structure were measured, and every sample underwent three heating and cooling experiments. The resistance change was recorded with each measurement in steps of 10 °C, and the temperature was maintained at each step of 10 °C for 25 min to observe the change in resistance. The test structure of the BN/graphene/BN pressure sensor with a microcavity is shown in As a temperature-sensitive material, BN/graphene/BN has a higher temperature coefficient, and can be used as a graphene pressure sensor temperature compensation module. The BN/graphene/BN pressure sensor with a microcavity has a positive temperature coefficient, high temperature sensitivity, and poor linearity, with a linear standard error of 2.66046. The BN/graphene/BN pressure sensor without a microcavity has a negative temperature coefficient, and low temperature sensitivity relative to the cavity, but it has high linearity, with a linear standard error of 0.27858. A comprehensive comparison of the BN/graphene/BN pressure sensors shows that the sensor without a microcavity has better temperature characteristics, apart from the graphene pressure sensor for temperature compensation. The visualized parameters of the BN/Gra/BN pressure sensors with and without a cavity that we designed as well as other temperature sensors are shown in −1, whereas for the BN/graphene/BN pressure sensor without a microcavity, the thermal expansion effect is smaller than the influence of electrophonon coupling, and the sensor temperature coefficient is negative, with a value of −0.16%°C−1. The two different structures present contrasting conclusions. The BN/graphene/BN pressure sensor with a microcavity shows good temperature sensitivity and the BN/graphene/BN pressure sensor without a microcavity shows good linearity. These results indicate that the influence of temperature on the pressure sensors based on BN/graphene/BN heterostructures should be considered, especially for devices with pressure microcavities. BN/graphene/BN heterostructures without microcavities can be used as high-performance temperature sensors, facilitating the design of a high-temperature graphene pressure sensor.For two different structures of BN/graphene/BN sensors, this article presents an analysis of the thermal expansion and the influence of electrophonon coupling on the temperature properties of graphene. When the effect of thermal expansion is greater than that of electrophonon coupling, the resistance of the BN/graphene/BN pressure sensor increases with an increase in temperature. The results show that, for the BN/graphene/BN pressure sensor with a microcavity, the thermal expansion effect is greater than the influence of electrophonon coupling, and the sensor temperature coefficient is positive with a value of 0.25%°C