Invariant spatial context can guide attention and facilitate visual search, an effect referred to as “contextual cueing.” Most previous studies on contextual cueing were conducted under conditions of photopic vision and high search item to background luminance contrast, leaving open the question whether the learning and/or retrieval of context cues depends on luminance contrast and ambient lighting. Given this, we conducted three experiments (each contains two subexperiments) to compare contextual cueing under different combinations of luminance contrast (high/low) and ambient lighting (photopic/mesopic). With high-contrast displays, we found robust contextual cueing in both photopic and mesopic environments, but the acquired contextual cueing could not be transferred when the display contrast changed from high to low in the photopic environment. By contrast, with low-contrast displays, contextual facilitation manifested only in mesopic vision, and the acquired cues remained effective following a switch to high-contrast displays. This pattern suggests that, with low display contrast, contextual cueing benefited from a more global search mode, aided by the activation of the peripheral rod system in mesopic vision, but was impeded by a more local, fovea-centered search mode in photopic vision. T-shaped target item amongst L-shaped distractors. Unbeknown to the observers, some of the search displays are repeatedly presented with the items arranged in an invariant spatial layout. Results show that observers can capitalize on those “hidden” regularities to facilitate search, compared with novel item arrangements . Thus, while spatial contexts in real environments are greatly affected by changes in ambient lighting and luminance contrast, the impact of these changes on contextual cueing has, to our knowledge, never been systematically examined. Rather, the acquisition of spatial context cues has been examined almost exclusively in relatively constant, standard laboratory lighting conditions —the implicit assumption being that the contextual-learning effects demonstrated under these conditions generalize across—that is, are independent of— variations in scene lighting and contrast factors. Clearly, however, this independence assumption—and, thus, the robustness of visual context learning and memory—must be verified empirically. In fact, there are reasons to doubt the validity of this assumption given that, for instance, changes in ambient lighting between day and night bring different visual-sensory systems into play , potentially affecting the extraction and thus the learning of structural relations and/or the recall of successfully acquired contextual regularities. Note that establishing in-/dependence of contextual cueing from variable environmental conditions would be not only of theoretical importance but also potentially of relevance for contextual training in certain real-world scenarios, such as driving simulation. On these grounds, the present study was designed to examine whether and how long-term statistical learning of spatial regularities, or “context cues,” in our visual environment depends on the prevailing ambient-lighting and luminance-contrast conditions, and whether and to what extent contextual cues acquired under certain lighting and contrast conditions could be effectively transferred to  changed environmental conditions.Before elaborating the precise hypotheses tested in our study, it is useful to provide a brief review of the differential impacts of photopic (daylight) and mesopic/scotopic (low lighting/nightlight) vision on the “visual span”  would exert the same influences on contextual cueing. In the present study, we investigated the role of these factors in a “traditional” contextual-cueing paradigm with meaningless, artificial stimuli, which afford greater control of display variables compared with naturalistic scenes . This is not to say that the findings necessarily extend one-to-one to naturalistic scenes, which, qua being meaningful, provide additional cues deriving from “scene grammar”  contextual cues. Of note, we examined not only for contextual learning under certain lighting and stimulus contrast conditions but also for the transfer of any acquired cueing effects from the  learning to a (subsequent) test session with (in some experiments) unchanged and (in others) changed lighting and contrast conditions. We specifically hypothesized that, compared with high-contrast stimuli, low-contrast stimuli presented under conditions of photopic vision would narrow the visual span, thus hampering contextual learning and/or retrieval. By contrast, under mesopic conditions, discrimination sensitivity for low-contrast displays would be boosted by activation of the rod system, expanding the visual span and thus facilitating contextual cueing of low-contrast contexts compared with photopic vision. In addition, transition from high-luminance  to low-luminance contrast (during testing) or from mesopic to photopic vision with low-luminance contrast may narrow the visual span, hampering contextual retrieval.To test these hypotheses, we designed three experiments  with high-contrast and, respectively, low-contrast search displays presented in photopic and mesopic environments, to examine for differential contextual-learning effects  and transfer effects (in the subsequent transfer sessions). In particular, as almost all published contextual-learning studies have been conducted in photopic vision, we focused on contextual learning under mesopic vision conditions. Figure T-shaped target presented amongst L-shaped distractors. To examine for contextual learning and transfer effects across high/low stimulus contrasts and photopic/mesopic lighting environments, each experiment was divided into an initial training phase followed by a transfer phase. During training, the luminance contrast was set to high in Experiments 1A, 2A, and 3A, and to low in Experiments 1B, 2B, and 3B; in the transfer phases, the luminance contrasts were swapped in Experiments 1–2, but remained the same as in the training phases in Experiment 3 .Experiment 1 was designed to investigate the influence of item-background contrast on contextual learning and transfer under photopic vision conditions. To this end, the display contrast was set to high in the training session but to low in the subsequent transfer session in Experiment 1A. Conversely, in Experiment 1B, the display contrast was set to low in the training session, but set to high in the transfer session , respectively. All participants had normal or corrected-to-normal visual acuity. The sample size was estimated by a power analysis using G*Power  Psychology Department in accordance with the Declaration of Helsinki, and the procedures were carried out in accordance with the relevant guidelines and regulations. 2). The viewing distance was fixed at 57 cm, maintained with the support of a chin rest. Stimulus presentation was controlled by using Psychtoolbox . The monitor brightness and contrast was set to 50%, and the cabin was lit by a ceiling lamp to a photopic level of environmental lighting  and 15 L-shaped distractors . Similar to previous studies  at the line junctions, making the Ls more similar to the target T. Each stimulus subtended 1.0° of visual angle. All search items were randomly presented on four concentric (invisible) circles with radii of 2°, 4°, 6°, and 8° of visual angle, respectively. Targets appeared only on the second or the third circle, while distractors could appear on all the four circles; this item arrangement is identical to that used in previous contextual-cueing studies  were presented on a dark-gray background (1.76 cd/m2). In Experiment 1A, the luminance of the search items was set to high (25.38 cd/m2) during the training session, and to low (2.33 cd/m2) during the transfer session. Conversely, in Experiment 1B, search-item luminance was low (2.33 cd/m2) during the training session, but high (25.38 cd/m2) during the transfer session. We calculated the display contrast  in terms of the Michelson contrast, which is defined as(Ii − Ib)/(Ii + Ib), where Ii and Ib represent the luminance of the search items and the background, respectively.As illustrated in Fig. T and the Ls) were kept constant and repeated once per block during the experiments; for the new displays, by contrast, the locations and orientations of the distractors were randomly determined for each presentation. To maintain comparable repetitions of the target locations for both old and new displays, targets also appeared at eight predefined locations in the new displays. The orientation of the target (leftward pointing vs. rightward pointing) was randomly selected for each search display, whether new or old, thus preventing any RT advantage from constant target orientation in old versus randomly variable orientation in new displays.Both experiments consisted of a training session of 25 blocks, followed by a transfer session of five blocks and a recognition test. Each block consisted of 16 trials, eight with “old” and eight with “new” displays, presented in randomized order. For the old displays, the locations of all the search items (both the T amongst the distractors Ls and discern its orientation (leftward pointing or rightward pointing), as rapidly and accurately as possible, by pressing either the left or the right arrow key on the keyboard with their left or right index finger, respectively. Each trial started with the presentation of a central fixation cross for 800–1,000 ms, followed by a search display that remained on the screen until a response was made or until the presentation exceeded 10 seconds. The next trial started after a random intertrial interval (ITI) of 1.0–1.2 seconds.Participants were instructed to find the target Prior to performing the visual search task, participants practiced the task in one block of 16 trials, in which the luminance contrast of the stimuli and the ambient lighting were set to the more challenging settings used in the training or the test session . This was done to ensure that participants were able to actually perform the task under the more difficult conditions—the assumption being that if participants reached an accuracy greater than 75% in the more difficult practice condition, they would also be able to perform the task under the easier condition. Participants who performed worse than 75% correct practiced the task again for two or three blocks, until they reached the accuracy criterion. No participants were excluded based on the current criteria. The item configurations displayed in the practice session were not used in the later parts of the experiment. Participants were free to take a break between blocks.After the formal visual search experiment , participants received one block of the recognition test, consisting of eight old-display and eight new-display trials. Participants had to make two-alternative forced choices as to whether a given display was an “old” or a “new” one by pressing the left or the right arrow key, respectively. The ambient lighting condition and the display contrast were the same as was used during the training sessions. The same dark-adaptation procedure was adopted for the recognition test in the mesopic environment. Of note, participants were explicitly informed before the recognition test that half of the recognition displays were old while the other half were new.BF) with JASP (0.11.1), using the default Cauchy settings . Likewise, for Bayesian t tests, we used the default Cauchy prior . All Bayes factors reported for ANOVA main effects and interactions are “inclusion” Bayes factors calculated across matched models. Inclusion Bayes factors compare models with a particular predictor to models that exclude that predictor. That is, they indicate the amount of change from prior inclusion odds  to posterior inclusion odds. Using inclusion Bayes factors calculated across matched models means that models that contain higher-order interactions involving the predictor of interest were excluded from the set of models on which the total prior and posterior odds were based. Inclusion Bayes factors provide a measure of the extent to which the data support inclusion of a factor in the model. BF values less than 0.33 are taken to provide substantial evidence for the null hypothesis . To establish whether critical nonsignificant effects favor the null hypothesis, we calculated Bayes factors  = 23.79, p < .001, ηp2 = .45; Experiment 1B, F = 34.80, p < .001, ηp2 = .55. This effectively rules out a trade-off between the accuracy and speed of responses. Accordingly, trials with response errors and failures to respond were excluded from further RT analysis.The proportion of trials with response errors and response failures  were low . To examine for potential speed–accuracy trade-offs, we grouped the trials into four quartile subsets according to the response times (RTs) and examined the respective (quartile-subset) error rates. This analysis revealed that most errors were made with the slowest responses  into an “epoch” , yielding five task epochs for the training session and one epoch for the transfer session. Figures F = 5.81, p = .022, ηp2 = .17, with an overall RT facilitation of 160 ms for the old versus new displays. The effect of epoch was also significant, F = 22.07, p < .001, ηp2 = .43: response speed increased across the training session (436 ms faster RTs in Epoch 5 vs. Epoch 1), indicative of procedural learning . The significant Context × Epoch interaction, F = 2.54, p = .044, ηp2 = .08, indicates that the contextual-cueing effect developed as the experiment progressed , the training session of Experiment 1A replicated the standard contextual cueing effect, see Fig. .t test, with context (old vs. new) as a factor, showed nonsignificant result, t(29) = 0.64, p = 0.53, Cohen’s d = 0.12, BF10 = 0.24, indicating reduced contextual cueing. Note that the mean RT was still 72 ms faster to old versus new configurations, yet due to large interparticipant variation, this difference was not significant . To examine the transfer effect of contextual cueing, we further estimated the change of contextual cueing from the last block of the training session  to the test session  for each experiment. Given that the changes of the lighting and stimulus-contrast conditions between the training and test sessions had a substantial impact on general response speed, we calculated the transfer effect based on the relative contextual-cueing magnitudes . Figure t test revealed the reduction of the cueing effect (−7.36%) to be significant, t(29) = 2.11, p = .043. Together with the nonsignificant cueing effect in the test session (see above), this indicates that contextual cues extracted and learned from high-contrast displays (in the training session) could not be effectively transferred to low-contrast displays—that is, presenting search displays with low item-background contrast in daylight conditions impedes the expression of (acquired) contextual cueing.For the subsequent transfer session of Experiment 1A, in which the item-background contrast was switched from high to low, a paired-sample F = 0.38, p = .54, ηp2 = .01, BFincl = 0.21, nor the Context × Epoch interaction, F = 1.41, p = .23, ηp2 = .05, BFincl = .088, turned out to be significant, with the Bayes factor favoring the null hypothesis of no contextual cueing during the training session (though RTs were 44 ms faster for old than for new displays). Only the main effect of epoch was significant, F = 22.02, p < .001, ηp2 = .43, again reflecting significant procedural learning (552 ms shorter RTs in Epoch 5 compared with Epoch 1). Interestingly, however, while there was only numerical contextual facilitation in the (low-contrast) training session , a significant cueing effect emerged following the switch to high-contrast displays in the transfer session, t(29) = 2.84, p = .008, Cohen’s d = 0.52: There was an RT advantage of 280 ms for repeated versus nonrepeated displays. However, the change in the relative measure was only marginal  = 1.87, p = .07, BF10 = 0.904. Given that a substantial contextual-cueing effect was already evident in the first block of the transfer session , the invariant spatial context was likely acquired in (and transferred from) the training session, rather than reflecting a relearning effect developed in the transfer session. However, the low-contrast setting in the training session may have limited the expression of contextual cueing.By contrast, when presenting low-contrast displays under photopic conditions in the training session in Experiment 1B see Fig. , neitherTaken together, for conditions of photopic lighting, the findings of Experiment 1A revealed successful contextual learning under the high-contrast condition, but this acquired contextual facilitation could not be transferred to the low-contrast condition; by contrast, the findings of Experiment 1B suggest that repeated spatial arrangements could be successfully learned with low-contrast displays, but contextual facilitation was expressed only when the display contrast changed to high. The findings of Experiments 1A and 1B suggest that visual search displays encountered in daylight conditions at low item-to-background contrast impede contextual retrieval, but not contextual learning. Previous studies had shown that the visual span contracts with decreasing display contrast  and with appropriately adjusted screen-background and stimulus luminance settings.Experiment 2 was designed to examine the influence of display contrast on contextual cueing in mesopic vision. The experimental details were essentially the same as in Experiment 1, except that participants performed the tasks in a dark environment, with all light sources, apart from the display CRT, eliminated , respectively. Note that two participants had to be replaced in Experiment 2B because they failed to achieve the accuracy criterion on the practice trials .2). Before the start of the experiments, participants were given 10 minutes for dark adaptation . The item-to-background contrast was changed from high (0.76 cd/m2) in the training session to low (0.013 cd/m2) in the transfer session in Experiment 2A, and from low to high in Experiment 2B. The low-luminance contrast was calculated via multiple measures of luminance and curve fitting. This is because, under mesopic vision conditions, the luminance levels of the background and the search items were close to the lower measurable boundary of the Minolta Chroma Meter CS-100 (0.01 cd/m2). To reduce measurement noise, we took multiple measures and used curve fitting to obtain the luminance-characteristic curve of the LACIE CRT monitor from the RGB range between  to  (with display contrast set to 20% and brightness to 0%). Based on this, we extrapolated the luminance with RGB values of  (setting of the background) and of  (setting of the search items). This yielded estimated values of 0.008 cd/m2 and 0.013 cd/m2 for the background and the search items, respectively, for the low-contrast displays. After the dark adaptation, all participants reported that they could see the search items.To reduce the background lighting of the CRT screen to mesopic level, the monitor brightness was set to 0% and the contrast to 20%  = 10.11, p < .001, ηp2 = .26; Experiment 2B, F = 30.35, p < .001, ηp2 = .51—with error rates being increased for the slowest RTs, ruling out any trade-off between the accuracy and speed of responses. Accordingly, trials with response errors and failures to respond were excluded from further RT analysis.Similar to Experiment 1, response-error rates  and response-failure rates  were low. Again, we grouped the trials into four quartile subsets according to the RTs and examined the (quartile-subset) error rates in one-way repeated-measures ANOVAs. The pattern was similar to Experiment 1  = 38.49, p <.001, ηp2 = .57; 412 ms faster responding in Epoch 5 versus Epoch 1, and the context, F = 4.78, p = .037, ηp2 = .14; 81-ms RT advantage for repeated versus nonrepeated displays. But the Context × Epoch interaction was not significant, F = 1.49, p = .21, ηp2 = .05, BFincl = .079. When the display contrast was switched to low in the transfer session, contextual facilitation was 110 ms numerically, but failed to reach significance, t(29) = 1.27, p = .21, Cohen’s d = 0.23, BF10 = 0.40; 2,630 versus 2,740 ms for repeated versus nonrepeated displays. Consistent with this pattern, the relative change of normalized contextual cueing between the training and the test session in Experiment 2A  = −1.55, p = .13, BF10 = 0.59. That is, while contextual cueing differed little between the training and test sessions, the uncertainty engendered by the luminance reduction in the test session rendered the expression of contextual cueing somewhat noisy.The contextual-cueing effects for Experiments 2A and 2B are depicted in Figs. see Fig.  was nonsF = 15.87, p < .001, ηp2 = .35 (686 ms faster RTs in Epoch 5 vs. Epoch 1), and context, F = 4.57, p = .04, ηp2 = .14 (150-ms RT advantage for repeated vs. non-repeated displays), as well as the Context × Epoch interaction, F = 3.40, p = .01, ηp2 = .11. Furthermore, this successfully established cueing effect was transferred to the test session, in which the display contrast was switched to high (mean RT facilitation of 380 ms)—as confirmed by an ANOVA, with context as the main factor for the test session: F = 20.86, p < .001, ηp2 = .42, t(29) = 4.57, p < .001, Cohen’s d = 0.84. In fact, the switch from low to high stimulus contrast led to a significant increase of the relative cueing effect, by 7.38%, t(29) = 2.82, p = .008. Note that, with 423 ms, the cueing effect was already very substantial in the first block of the transfer session . Accordingly, the increase in the relative magnitude of contextual cueing indicates that high stimulus contrast promotes the expression (rather than just the acquisition) of contextual cueing.In Experiment 2B, participants also performed the task in the mesopic environment, but display contrast was switched from low during training to high during (transfer) test. Under these conditions, for the training session, the results were different to Experiment 1B : Participants showed contextual cueing with low-contrast displays in a mesopic environment. Both main effects were significant: epoch, Taken together, the results of Experiment 2 indicate that contextual cues can be successfully learned with both high-contrast and low-contrast displays under mesopic vision. However, switching stimulus contrast from high to low (as in Experiment 2A) makes the expression of contextual facilitation more variable, resulting in an overall large yet nonsignificant contextual-cueing effect in the test session. This pattern is similar to Experiment 1A, in which a change from high to low contrast also rendered a noisy expression of contextual cueing, under photopic vision conditions. Both experiments (1A and 2A) suggest that switching stimulus contrast from high to low brings about a system adaptation to the change that involves a narrowing of the visual span, which makes contextual retrieval (activation of learnt context cues by display contents) more noisy and impedes the expression of the contextual cueing. This is consistent with Zang et al.’s  demonstrComparison of Experiment 1B (photopic low to high contrast) with 2B (mesopic low to high contrast) indicates that contextual facilitation was overall increased, by some 100 ms, in mesopic vision, as compared with photopic vision, for both the training (150 ms vs. 44 ms) and the transfer sessions (380 ms vs. 280 ms). As shown by previous studies, in mesopic/scotopic vision, detection sensitivity is shifted toward the periphery and the visual system extracts information from a wider region around the current fixation point  would be available for contextual guidance under conditions of photopic vision.Two groups of 30 participants took part in Experiments 3A and 3B , respectively.The experimental paradigm was essentially the same as in Experiments 1 and 2, except for the following differences: In both experiments, display contrast was fixed for the training and test sessions , while the environmental lighting was changed from mesopic to photopic. Participants were initially given 10 minutes for dark adaptation .F = 3.57, p = .017, ηp2 = .45; Experiment 3B, F = 23.79, p < .001, ηp2 = .45, thus effectively ruling out speed–accuracy trade-offs.Similarly, participants mean response errors and response failures were low . More errors were made with slower responses, as confirmed by one-way repeated-measures ANOVAs yielding significant main effects of RT quartile-subset: Experiment 3A, F = 43.61, p < .001, ηp2 = .60 (431 ms faster RTs in Epoch 5 vs. Epoch 1); context, F = 8.74, p = .006, ηp2 = .23 (mean facilitation effect of 138 ms); and Context × Epoch interaction, F = 4.46, p = .002, ηp2 = .13. Importantly, the contextual-cueing effect was maintained—with an overall facilitation of 139 ms—in the transfer session in which the ambient lighting was switched from mesopic to photopic, t(29) = 3.24, p = . 003, Cohen’s d = 0.59. In fact, there was no evidence of a difference in the relative cueing effects between the training and test sessions  = 0.489, p = .63, BF10 = .22. In the test session, the mean facilitation effects were 117, 172, 128, 127, and 152 ms, respectively, in the five transfer blocks—likely reflecting successful transfer of the cueing effect from mesopic to photopic vision (rather than fast reacquisition under the latter condition).The contextual-cueing effects for Experiments 3A and 3B are depicted in Figs. F = 23.29, p < .001, ηp2 = .45 (790 ms faster RTs in Epoch 5 vs. Epoch 1), the main effect of context was significant, F = 4.73, p = .038, ηp2 = .14, with an overall facilitation effect of 138 ms. The Context × Epoch interaction failed to reach significance, F = 1.56, p = .19, ηp2 = .05, BFincl = .052. When the ambient lighting was switched from mesopic to photopic, the contextual-cueing effect was only marginal, t(29) = 2.01, p = .054, Cohen’s d = 0.37, BF10 = 1.13, averaging 163 ms across the whole transfer epoch —though, measured in terms of the relative effect, it remained equivalent to that in the training session  = −0.304, p = .76, BF10 = .203. In other words, contextual facilitation acquired with low-contrast displays remained unchanged when switching from mesopic to photopic vision.Experiment 3B examined for contextual learning and transfer with low-contrast displays, with a change from a mesopic to a photopic environment. The results of the training session were similar to the “equivalent” Experiment 2B (mesopic training with low-contrast displays): apart from a significant effect of epoch, In line with Experiment 2, Experiment 3 confirmed that invariant contexts, whether encountered under high or low stimulus-contrast conditions, could be effectively acquired and retrieved in mesopic vision. In addition, the results of Experiment 3 showed that learned contexts can be transferred from dark mesopic to bright photopic environments when display contrast remains high. However, the contextual-cueing effect was rendered statistically marginal when the display contrast remained low after the switch to photopic vision.All participants were administered a recognition test after the visual-search (training and test) sessions of each experiment. Prior to the recognition test, participants were explicitly told that they would be presented with equal numbers of old and new displays. Given this information, participants who provided the same, biased response  throughout the entire recognition session were excluded from further analysis. Altogether, three participants from Experiment 2B and one participant from Experiment 3A were excluded.t(29) = 3.67, p = .001, Cohen’s d = 0.37; Experiment 1B, 58.8% versus 46.3%, t(29) = 2.29, p = .03, Cohen’s d = 0.42; Experiment 2A, 60.0% versus 54.2%, t(29) = 1.41, p = .17, Cohen’s d = 0.26, BF10 = 0.47; Experiment 2B, 56.5% versus 47.7%, t(27) = 1.78, p = .087, Cohen’s d = 0.34, BF10 = 0.81; Experiment 3A, 61.2% versus 52.6%, t(28) = 1.93, p = .064, Cohen’s d = 0.36, BF10 = 0.99; Experiment 3B, 56.2% versus 55.0%, t(29) = 0.23, p = .82, Cohen’s d = 0.04, BF10 = 0.20. We further examined the old-/new-display discrimination sensitivities (d') and the associated response biases (c) per experiment  = 3.66, p = .001, mean effect of .44; Experiment 1B, t(29) = 2.32, p = .027, mean effect of .35, but not in the mesopic condition , providing some evidence of explicit learning under photopic conditions (in Experiments 1A and 1B), consistent with the finding of previous studies : 0.21 versus 0.39, F = 2.02, p = .157, BF10 = .435. Thus, from the present data, there is insufficient evidence to firmly conclude that contextual learning involves a greater degree of “explicitness” under photopic as compared with mesopic vision conditions.For the remaining data, participants’ mean hit and false-alarm rates were calculated, which turned out as follows: Experiment 1A, 62.5% versus 46.67%, t tests of the response-bias scores c , but not in Experiments 1B and 2B . There is no clear pattern in the bias scores across the six subexperiments, except that, in general, participants tended to respond rather conservatively  in the recognition test .A second group of s c Fig.  reached The present study investigated the acquisition of spatial context cues and their retrieval with high-contrast and low-contrast displays under photopic and mesopic ambient-light conditions. The results revealed differential effects of luminance contrast on contextual learning and retrieval. Contextual cueing was effective with high-contrast displays, but was multifaceted with low-contrast displays. Low-contrast contexts could be learned under both mesopic and photopic vision, but were retrievable only under mesopic conditions. Learned context cues were transferred from low-contrast to high-contrast conditions, but not vice versa. When the environmental lighting was switched from mesopic to photopic, retrieval of low-contrast contexts was only marginally significant. Taken together, our findings suggest that low-contrast contexts can be used much better in mesopic than in photopic vision; and display contrast plays a critical role for contextual retrieval in photopic vision.The finding that contextual retrieval works only with high-contrast (but not low-contrast) displays in the photopic environment is theoretically important, given that contextual cueing is thought to be a robust phenomenon, manifesting in various complex environments , although contextual cueing was not significant with the low-contrast displays in the training session, contextual facilitation manifested already in the first block of the transfer session with the high-contrast displays, suggesting that contextual learning did take place in the preceding training session. In other words, in the photopic environment, low-contrast displays allow for contextual learning, while affecting the expression of contextual cueing. This is similar to the findings with gaze-contingent displays with a restricted foveal view of two to three items . When performing search in photopic vision, observers tend to shift attention overtly to different locations in the search display by making saccades and then focus processing on the immediate region represented within and around the fovea to find the target  can aid contextual retrieval. With regard to the present study, we propose that in mesopic vision, the effective contextual learning and retrieval of low-contrast contexts is attributable to the more global search mode supported by the  rod system. In photopic vision, by contrast, contextual retrieval was likely hampered by the more local search mode induced by low-contrast displays. Interestingly, repeated contexts could still be learned in local search mode, as evidenced by the significant cueing effect manifesting already in the first block of the transfer session, after the switch of the display contrast switched from low to high. This is consistent with previous findings  and mesopic vision (significant cueing in Experiment 2B). Previous studies have shown that the availability of the global spatial configuration plays an important role in contextual learning and retrieval  displays cause more variability in the learning  and retrieval phases (Experiments 1A and 2A), rendering contextual-cueing effects nonsignificant; similarly, environmental changes introduce variability in the availability of context cues , rendering the cueing effect statistically “marginal.” However, while the variability account provides an explanation of the effect pattern on an information-processing level,What do our findings tell with regard to the question of whether contextual cueing is based on an explicit or an implicit memory system? It is important to note that explicit knowledge of repeated displays does not guarantee a significant contextual-cueing effect (see Results of Experiments 1B and 2B), suggesting that the cueing effects in visual search task and observers’ ability to explicitly discriminate repeated from non-repeated displays may be decoupled. It has long been debated and remains controversial whether automatic contextual guidance and conscious recognition are based on a single memory system that operates in a two-stage process  rod system in mesopic vision may help both the learning and the retrieval of low-contrast contexts. One possible limitation of the present study, though, is that we tested (only) six combinations of luminance contrast and environmental lighting in the training and test sessions, leaving other potentially important conditions untested, such as contextual learning in the photopic environment and retrieval in the mesopic environment while fixing the display contrast at either low or high. Yet the outcomes from the combinations that we tested allow us to predict, with confidence, what would happen under these conditions. Specifically, contexts encountered at high stimulus contrast can be successfully learned  and retrieved (Experiments 2B and 3A) not only in photopic environments  is compromised with low-contrast displays under photopic conditions, it is effective under mesopic conditions. These differential effects are likely attributable to differential search modes in mesopic versus photopic environments: A more local, fovea-centered mode in photopic vision hampers contextual retrieval with low-contrast stimuli, but helps explicit recognition of repeated displays; in contrast, a more global search mode in mesopic vision aids contextual cueing, but may impede explicit recognition. Thus, effective contextual learning and retrieval depend on the search mode induced by the ambient lighting, and specifically a global mode mediated by the rod system coming into play under conditions of mesopic vision. Concerning the latter, our finding of successful contextual learning and transfer from low to high display contrast in the mesopic environment may have interesting implications for contextual learning in real-life (applied) scenarios. For example, complex road layouts (for navigation in both night and daylight conditions) may actually be more efficiently learned if these are encountered under low-luminance night-vision conditions first.This research is supported by the cultivation project of the province-leveled preponderant characteristic discipline in the college of education of Hangzhou Normal University (18JYXK027) and , the Starting Research Fund from Hangzhou Normal University (RWSK20190315), the National Natural Science Foundation of China (NSFC 31600876) to X.Z., and the German Research Foundation  to Z.S. The funding bodies play no role in the study design, data collection, analysis, decision to publish, or preparation of the manuscript.https://github.com/msenselab/mesopic_contextual_cueing).The data and codes for all experiments are available (


This combination of materials allows for a high-quality modulator device with high performances: a ~39 GHz bandwidth (BW) with a three-fold increase in modulation efficiency compared to previously reported high-speed modulators. This 2D-3D dielectric integration paves the way to a plethora of electronic and opto-electronic devices with enhanced performance and stability, while expanding the freedom for new device designs.Electro-absorption (EA) waveguide-coupled modulators are essential building blocks for on-chip optical communications. Compared to state-of-the-art silicon (Si) devices, graphene-based EA modulators promise smaller footprints, larger temperature stability, cost-effective integration and high speeds. However, combining high speed and large modulation efficiencies in a single graphene-based device has remained elusive so far. In this work, we overcome this fundamental trade-off by demonstrating the 2D-3D dielectric integration in a high-quality encapsulated graphene device. We integrated hafnium oxide (HfO Here, three-dimensional hafnium oxide and two-dimensional hexagonal boron nitride are integrated in the insulating section of double-layer graphene optical modulators, leading to a maximum bandwidth of 39 GHz and enhanced modulation efficiency. Although Si photonics is nowadays a prime candidate to fulfill these requirements3, graphene is rapidly becoming a major contender in several optoelectronic applications, such as ultrafast modulators5 and silicon-integrated photodetectors7. Graphene-based modulators have already proven broadband optical bandwidth1, high speed9, relatively high modulation efficiencies10, and temperature stability8. These devices are all based on complementary metal–oxide–semiconductor (CMOS)-compatible material13, where CMOS design and fabrication techniques can be further leveraged to decrease costs. However, graphene-based modulators are yet to demonstrate all operation requirements at once. More specifically, EA graphene modulators struggle to show high-speed and high modulation efficiencies simultaneously14. This bottleneck is mostly due to the weak graphene/dielectric combination and the limited quality of graphene.Broadband optical modulators with ultra-high-speed, low-drive voltage, and hysteresis-free operation are key devices for next-generation datacom transceiversκ dielectrics lie at the core of its success, 2D dielectrics are hindering the development of graphene- and other 2D-based electronics and optoelectronic devices15 and are clearly outperformed by traditional 3D high-κ dielectrics. This underperforming 2D-dielectric/graphene combination deepens even further the fundamental trade-off between speed and modulation efficiency inherent to the double-layer (DL) modulators14. In the DL architecture, the overlapped top and bottom graphene electrodes act as a capacitor (C). The larger the C, the higher the modulation efficiency. On the other hand, the speed of the modulator defined as f3dB = 1/(2πRC) is inversely proportional to C . In this framework, the quality of graphene appears as a valid turnaround to overcome this fundamental limitation. High electron mobility is expected to minimize the overall resistance and reduce the insertion loss (IL)9, thus increasing the bandwidth and the extinction ratio (ER). However, the quality of graphene is very sensitive to its environment, e.g., the dielectric to encapsulate it. Indeed, no graphene/dielectric combination has been able to ensure high charge carrier mobilities and low levels of residual doping in existing graphene waveguide-coupled modulators16. The growth of nonlayered  dielectrics, e.g., aluminum oxide (Al2O3), silicon nitride  modulation efficiency as high as 2.2 and 1.49 dBV−1, respectively, a f3dB bandwidth of ~39 GHz, and a device footprint of 60 μm × 0.45 μm ≈ 27 μm2 . Moreover, the hBN–HfO2–hBN-based devices show a symmetric and nearly hysteresis-free operation. The larger breakdown voltage of this 2D–3D dielectric, even beyond the full transparency regime , increases the ER and reduces the IL of the modulators.In this work, we demonstrate the 2D–3D integration of hBN and HfO20 formed by two grating couplers21 feeding light in and out of an optical waveguide . This low dielectric constant and reduced breakdown voltage  and contact resistivity (ρc) for a graphene modulator with the same geometry and dielectric combination as the device in Fig. μ ≈ 12,000 cm2 V−1 s−1  and the contact resistivity ρc ≈ 800 Ω μm achieved experimentally  driving the modulator with a 2C = 52 fF is the capacitance between the top and bottom graphene electrodes and VAC = 3.5 V the voltage swing12. This value of energy per bit is on par with state-of-the-art SiGe technologies34.Like the speed of the modulator, the power consumption understood as the switching energy per bit also benefits from the small footprint of the device. Ignoring the parasitic pad capacitance, we obtain for the modulator in Fig. EF  increasing (decreasing) as a function of EF9. In the full transparency regime  and dielectric strength (EBD) of any given dielectric. The square boxes in Fig. 2− and hBN-based modulators  and the black star represents the 2–hBN modulator of Fig. 37 (marked with dots) and from our dielectric characterization , achieving higher EF values than the hBN dielectric while equally preserving the intrinsic qualities of graphene.To directly compare modulators with different dielectrics, it is more convenient to compare the transmission as a function of −1 for device lengths L = 12, 24, and 42 μm, respectively. Once normalized by its length, we obtain 0.025, 0.054, and 0.047 dBV−1 μm−1. These results exceed the state-of-the-art modulation efficiency of 0.038 dBV−1 μm−110. Still, the premature hBN breakdown compromises the ER and the IL. Indeed, the measured ER = 0.75, 2.3, and 4.9 dB  than the device in Fig. L = 60 μm), the modulation efficiency is lower . Considering the breakdown capabilities of hBN–HfO2–hBN dielectric  or hybrid technologies like Si/indium phosphide39 (InP), Si/SiGe40, or InGaAlAs40 offer outstanding performances in modulator applications, those are either not scalable41 (LiNbO3) or their integration with a CMOS fabrication line remains challenging42. Nowadays, Si and graphene are envisaged as the most scalable, cost-effective, and CMOS-compatible materials for amplitude modulator applications1. To compare our results with state-of-the-art graphene and Si amplitude modulators, both EA and Mach–Zehnder interferometer configurations included, we summarize our results in Fig. f3dB bandwidth  growth on top of graphene, the integration of HfO2 in-between hBN prevented any damage to the underlying graphene and allowed clean graphene–hBN interfaces. These clean interfaces yielded a symmetric and nearly hysteresis-free operation. Moreover, this 2D–3D integration enabled full transparency while maintaining the high mobility and low doping of intrinsic graphene. More importantly, the hBN–HfO2–hBN-based EA modulators were able to reach high modulation speeds with strong modulation efficiencies, overcoming the fundamental limitations of the DL graphene configuration and outperforming state-of-the-art graphene and Si technologies. The compatibility of this hBN–HfO2–hBN dielectric with Si and other 2D materials might allow for considerable scaling improvements and greater device functionality in a broad range of graphene- and 2D-based electronic and optoelectronic applications, even beyond graphene-based modulators.In this work, we demonstrated the advantages of integrating hBN with a 3D high-20. For the fabrication of the electroabsorption modulator, the graphene and hBN flakes were exfoliated from highly oriented pyrolytic graphite and hBN crystals, respectively. The bottom hBN–graphene–hBN stacks were prepared by the van der Waals assembly technique23 and transferred directly onto the Si waveguide separated by a 10 nm spacer of high-quality thermal SiO2. The bottom hBN flake (separating the graphene and the SiO2 layer) thickness of ~5 nm was chosen to enhance the graphene absorption while isolating the graphene from the rough SiO2 substrate. The top hBN has a thickness of ~10 nm. The stack has been etched by reactive ion etching in an oxygen (O2) and trifluoromethane (CHF3) (4:40 sccm) environment to expose the graphene edge. The bottom stack was then contacted by a 3/15/30 nm Cr/Pd/Au metal combination. The 10 nm hafnium oxide film has been deposited at 250 °C prior depositions of a 2 nm sputtered SiO2 seed layer by ALD. Tetrakis-dimethylamido hafnium (TDMAH) (0.4 s purge time) and water vapor (5 s purge time) as precursors have been used in a Savannah G1 system from Cambridge Nanotech. The top hBN–graphene–hBN stack with a 7 nm- and 21 nm-thick bottom and top hBN layers has followed the same fabrication steps as the bottom stack.The Si photonic waveguide with a core cross-section of 750 nm × 220 nm was prepared on the IMEC iSiPP25G silicon-on-insulator platformSupplementary Information


Finally, the test characterization of graphene/TMDs hetero-structures was performed by AFM, SEM, EDX, Raman and PL spectroscopy to obtain and grasp the morphology and luminescence laws. The test results show that graphene/TMDs vdWs hetero-structures have the very excellent film quality and spectral characteristics. There is the built-in electric field at the interface of graphene/TMDs heterojunction, which can lead to the effective separation of photo-generated electron–hole pairs. Monolayer WS2 and MoS2 material have the strong broadband absorption capabilities, the photo-generated electrons from WS2 can transfer to the underlying p-type graphene when graphene/WS2 hetero-structures material is exposed to the light, and the remaining holes can induced the light gate effect, which is contrast to the ordinary semiconductor photoconductors. The research on spectral characteristics of graphene/TMDs hetero-structures can pave the way for the application of novel optoelectronic devices.The Van der Waals (vdWs) hetero-structures consist of two-dimensional materials have received extensive attention, which is due to its attractive electrical and optoelectronic properties. In this paper, the high-quality large-size graphene film was first prepared by the chemical vapor deposition (CVD) method; then, graphene film was transferred to SiO The size of traditional silicon-based metal oxide semiconductor (CMOS) transistors become smaller with increase in the chip integration, and the preparation processes of device become much more complicated, so researchers have begun to focus on the ultra-thin hetero-structure-based optoelectronics , 2. The Since the 2D-crystalline materials have the strong interactions against a light, they have attracted extensive attentions as photosensitive materials . Graphen2 and graphene/MoS2 hetero-structures were composed of three kinds of semiconductor materials with different dielectric constant, band gap width and absorption coefficient. The 2D materials were directly grown on the single crystal graphene film of SiO2/Si substrate to form the graphene/TMDs hetero-structures, which can ensure the clean interface and atomic level transition of hetero-structures. The structure of graphene, MoS2 and WS2 can be analyzed by the AFM, SEM, EDX, Raman and photoluminescence spectroscopy to master the spectral characteristics of graphene/TMDs hetero-structures, which can be used to prepare the high-speed electron mobility transistors (HEMT) and photoelectric detectors [In this paper, graphene/WSetectors –25.The large-area, high-quality graphene film was prepared by CVD system, which is composed of the tube furnace, gas mixing system and vacuum machine. First, the copper foil with a size of 10 cm × 10 cm was placed into 1 mol/L hydrochloric acid solution for the 3 min ultrasonic cleaning. Then, it was washed with water and ethanol in turn. Subsequently, it was dried by blowing argon gas. Finally, it was inserted in the middle of the quartz tube, and we installed the system and corrected the air pressure  2) gas were 1000 °C, 20 min and 30 sccm, respectively. At this time, the surface of copper foil would form the large area single crystal domain, and H2 gas can reduce copper oxide, which can obtain the high-purity copper substrate. The growth temperature remains constant while entering stage 2, the flow rate of H2 gas was adjusted to 10 sccm, meanwhile 35 sccm methane (CH4) gas was also introduced, the growth time and growth pressure were, respectively, maintained for 10 min and 1.08 Torr, and the growth rate of graphene was approximately 16 μm/s in our CVD experiment, which would ensure the preparation of relatively uniform monolayer graphene film [As we all know, polycrystalline copper foil would affect the quality of graphene, it is necessary to anneal the copper foil substrate before the growth experiment of graphene. The specific conditions of the annealing processes at stage 1 were the following: the annealing temperature, time and the flow rate of hydrogen 2(SO4)2 solution with a mass fraction of 3%, and the treatment time was 3–4 h. Then, the PMMA/graphene by the glass slide was rinsed repeatedly in deionized water 2–3 times, and PMMA/graphene was removed to the 50 °C constant temperature table by SiO2/Si substrate, which can remove the water vapor between monolayer graphene material and SiO2/Si substrate, and the monolayer graphene material can be better attached to SiO2/Si substrate. In this step, SiO2/Si substrate with a size of 1 × 1 cm2 was ultrasonically cleaned with acetone, ethanol and water for 15 min, and the surface of SiO2/Si substrate is very clean and uniform, which is conducive to the growth of graphene/TMDs hetero-structures. Finally, PMMA/graphene/SiO2/Si was put in acetone solution for 3–4 h to dissolve PMMA and repeatedly wash it with alcohol and deionized water to ensure that monolayer graphene film can be transferred to SiO2/Si substrate.The following describes the transfer-specific processes of monolayer graphene material to SiOubstrate . First, 2/Si substrate was used for the growth of MoS2 and WS2 material. The MoO3, WO3 and sulfur powders were used as the growth molybdenum source, tungsten source and sulfur source, respectively. The high-purity Ar gas was also used to prepare the graphene/MoS2 and graphene/WS2 hetero-structures, respectively. First, the quartz boat with 100 mg sulfur powder was placed at the upstream of tube furnace. Then, 2 mg MoO3 powder (or WO3 powder) was filled in another quartz boat, and the graphene/SiO2/Si substrate was flipped upside down on MoO3 powder (or WO3 powder). And then, the quartz boat equipped with the graphene/SiO2/Si substrate and MoO3 powder (or WO3 powder) was inserted into the high temperature area of tube furnace. The heating belt was wound on the quartz tube to heat the sulfur powder, which would ensure that sulfur powder was well controlled and uniformly evaporated, as shown in Fig. 2 and WS2 were 750 °C, 920 °C and 10 min, respectively. Meanwhile, the first-stage temperature was maintained at 100 °C for 10 min, which can remove the water vapor of tube furnace. The specific temperature change diagram is shown in Fig. 3 and WO3 powder, and the product was deposited on graphene/SiO2/Si substrate. Therefore, the growth rate of graphene/TMDs hetero-structures was consistent with that of TMDs materials [2 and WS2 material, the tube furnace was naturally cooled to room temperature, and the color of substrate becomes light yellow.In CVD dual temperature zone tube furnace, the graphene/SiOaterials . After tIn this paper, the test and characterization methods of graphene/TMDs hetero-structures mainly include the optical microscope (OM), Raman and photoluminescence (PL) spectroscopy, field emission scanning electron microscope (FESEM), energy-dispersive X-ray spectroscopy (EDX) and atomic force microscope (AFM). First, the surface morphology of graphene/TMDs hetero-structures can be observed by optical microscope, SEM and AFM. The layer number of hetero-structures can be judged according to the different contrast of hetero-structure sample. Then, the spectral characteristics of graphene/TMDs hetero-structures were tested and characterized. The growth morphology, growth pattern and growth mechanism of TMDs materials on graphene surface were analyzed and speculated based on the characterization results . Next, R−1–3000 cm−1 and 550–800 nm, respectively. And the Raman and PL spectra were 10% power and 5% power, respectively. The following were the specific test conditions, spectral resolution ≤ 0.65 cm−1; spatial resolution: horizontal ≤ 1 μm, vertical ≤ 2 μm; 532 nm laser, 50× objective lens ; scan time 15 s, and the cumulative number is 2.Both the PL and Raman spectra were collected by the LabRAM HR Evolution high-resolution Raman spectrometer, which was produced by HORIBA Jobin Yvon (French company) , 33. The2 hetero-structure under the different locations of SiO2/Si substrate. Since the color of graphene film transferred to SiO2/Si substrate was not much different, the graphene film is relatively uniform and complete. The surface of graphene/SiO2/Si substrate was clean except for a small amount of particles, which indicates the presence of better quality graphene film. Meanwhile, the nucleation density of WS2 became maximum when the gas concentration is sufficient in the growth experiment of WS2. And the WS2 grown on graphene/SiO2/Si substrate was the triangular structure grain with the uniform grain surface and a side length about 120 μm. The shape of WS2 was regular and complete, and the thickness was uniform, which is much larger than the size of mechanical peeling sample [2 sample is highly uniformly distributed, the triangular monolayer WS2 film has the higher quality and lower defects. It can be seen from Fig. 2 film sample is triangle, and the thickness of WS2 film is 0.83 nm, which indicates the preparation of monolayer WS2 film. In addition, SEM was also used to analyze the morphology of WS2 sample film, and the morphology was the regular triangular with uniform thickness, as shown in Fig. 2 hetero-structure material is successfully transferred and prepared.The morphology of hetero-structures can be distinguished by the high-resolution microscope of Raman spectrometer. Figure g sample . In Fig.2 sample can be effectively judged by the characteristic peak position and wave number difference of Raman spectra. Figure 2 sample at different positions, the E12g and A1g characteristic peaks were located at 350.4 cm−1 and 416.1 cm−1, respectively. When the bulk WS2 changes to monolayer material, E12g and A1g characteristic peaks appear the blue-shifted and red-shifted, respectively. Therefore, the layer number can be judged by the wave numbers difference between two characteristic peaks, and the wavenumber difference was 65.7 cm−1, so the triangular WS2 grains were monolayer material. In Fig. 2. As we all know, the PL intensity of 2D material is related to the crystal quality and layer number. The 2D material has the fewer defect and layer number, and the luminous intensity is higher, which indicates that the crystal quality is better [12g plane vibration mode remains basically unchanged with increase in the excitation power, and the A1g vibration mode between the planes moves to the short wave number direction. This is because the A1g vibration mode has the great relationship with electron concentration, and the increase in electron concentration would lead to the reformation of band gap. As shown in Fig. 2 increases with the laser power increases, and there exists the fluorescence quenching phenomenon, which is due to the reformation of band gap and the interlayer interaction of hetero-structures. At the same time, it can also be found that the local temperature of material did nearly not change with increase in the laser power. This is because WS2 is the atomic layer-level nanomaterial.The molecular vibration and rotation information of material can be obtained by Raman spectroscopy, which are the fingerprint vibration spectra for identifying the material structure. The layer number and crystal quality of WS−1, 1583 cm−1 and 2674 cm−1. The D peak is related to the disorder of graphene lattice structure, and the position of D peak was blue-shifted when graphene material has more lattice defects, which can reflect the defects and impurity content of crystal. The 2D peak is the two-phonon second-order resonance Raman peak, which can indicate the carbon atoms arrangement of graphene material. Besides, the G peak is caused by the E2g mode of first Brillouin zone center, the peak height increases almost linearly with the layers number of graphene, and the G peak intensity is related to the doping of graphene to a certain extent. The relative ratio of 2D peak and G peak can be used to roughly determine the layers number of graphene, and the ratio of D peak to G peak would decrease when the defect density is increased. The weak D peak appears in the Raman spectra of graphene when the growth of MoS2 (or WS2) material was completed, which indicates that the graphene domain still maintains the high quality. The 2D peak intensity of exposed graphene area weakened, which is affected by the high-temperature growth process. The full width at half maxima (FWHM) of graphene 2D peak gradually increases with increase in the layers number, and the peak position of 2D peak is blue-shifted, which may be related to the energy band relationship of graphene material. The electronic energy band structure splits with increase in the layers number, and a variety of phonon resonance scattering processes would occur. The exciton peak would be excited by absorbing more energy, which would lead to the blue shift of 2D peak position. The peak intensity of G peak at Point C and E is significantly higher than that of 2D peak. The I2D/IG ratio decreases with increase in the thickness, and the transferred graphene in this experiment was not very uniform, which is within the allowable range. Figure −1 and 2672 cm−1, and the intensity of two characteristic peaks differs greatly. Due to the interaction change between graphene and underlying SiO2, the characteristic peak ratio of I2D/IG is decreased. Meanwhile, there were no D defect peak of Raman spectra, which indicate that the selected graphene region has a high quality and the carbon atoms are highly ordered.The layer number characterization and quality information of graphene material can be obtained by Raman spectroscopy. In Fig. 2 hetero-structure material, and there were two spectra of 300 cm−1 ≤ ω ≤ 500 cm−1 and 1400 cm−1 ≤ ω ≤ 3000 cm−1, which were fitted by the Lorentz function. There were the E12g and A1g modes characteristic peaks of WS2 in the range of 300 cm−1 ≤ ω ≤ 500 cm−1. The E12g phonon mode is the in-plane displacement of sulfur and tungsten atoms, while the A1g phonon mode is the out-of-plane displacement of sulfur atoms, the above phonon mode locations and intervals vary with the layers number. The G and 2D peaks of graphene appear in the spectra region of 1400 cm−1 ≤ ω ≤ 3000 cm−1, and the layer number and crystal quality information of graphene can be obtained according to the intensity ratio and the peak position of characteristic peaks.Raman spectroscopy was used to characterize and analyze the graphene/WS2 hetero-structure at different points under the 532 nm laser. It can be found that the intensity of E12g characteristic peak was higher than that of A1g characteristic peak intensity, and the E12g and A1g characteristic peaks were located at 349.3 cm−1 and 417.1 cm−1, respectively. The Raman spectra 2D and G peaks of graphene/WS2 hetero-structure were, respectively, at 1591.5 cm−1 and 2680.9 cm−1, and the peak position of 2D and G peaks rise compared with pure graphene, which may be related to the effective interlayer coupling of WS2 nano-sheets and the strain effect generated by the high temperature heating during CVD growth. The Raman spectra of graphene/WS2 hetero-structure material is only the sum of the individual separated WS2 and graphene spectra, which can confirm the formation of vdWs heterojunction interface. The PL spectra intensity is related to the crystal quality and layer number. Raman spectroscopy focuses on the influence of hetero-structures formation on the vibration modes, and the electronic band structure of TMDs hetero-structures material can be mainly obtained by PL spectra. Figure 2 hetero-structure at different points. The strongest luminescence peak was located at 624 nm, and the corresponding band gap was 1.99 eV, which is consistent with the band gap width of monolayer WS2. The graphene/WS2 hetero-structure material at different positions has the different intensity and shape of PL spectra, and the crystal quality is not very good. Therefore, the preparation processes of hetero-structure need to be further improved. The PL spectra intensity of graphene/WS2 hetero-structure is weaker than that of WS2. This is because the inter-layer coupling of graphene/WS2 hetero-structure changes the exciton fluorescence of hetero-structures region, which would lead to the separation of electron–hole pairs and the reduction in fluorescence. Meanwhile, the peak position shifts when the graphene/WS2 hetero-structure is formed, and the transfer of charge can cause the shift of Fermi surface, which can make the free excitons change into the charged excitons. Figure 2 hetero-structure. The in-plane phonon mode E12g characteristic peak and the out-of-plane phonon mode A1g characteristic peak were, respectively, at 356 cm−1 and 418 cm−1, where the above characteristic peak is increased with increase in the laser power. The peak position and shape of characteristic peak were uniform within single crystal, and the electronic characteristics of WS2 on graphene/SiO2/Si substrate were uniform. The thickness of WS2 sheet can be determined according to the frequency difference between A1g and E12g characteristic peaks, and the average distance was 62 ± 0.2 cm−1, which is consistent with the thickness of monolayer WS2. Compared to the peak positions of intrinsic graphene, the G peak and 2D peak positions of graphene/WS2 hetero-structure from 1578.7 cm−1 and 2685.8 cm−1 change to 1582.2 cm−1 and 2689.5 cm−1, respectively. Besides, the intensity of G peak becomes stronger than that of 2D peak with increase in the laser power, and decrease in the I2D/IG ratio, which is caused by the interaction change between graphene and SiO2/Si substrate [2 hetero-structure is increased with increase in the laser power, the FWHM of PL spectra also increasing, and the shape of PL spectra is changed. The reason is that the test temperature around hetero-structure is increased, and there is also the strong interlayer coupling at the interface of graphene/WS2 heterojunction.The frequency difference of two different Davydov splitting peaks can reflect the interaction magnitude of vdWs hetero-structures. Therefore, the intra-layer vibrating phonon mode frequency of multilayer 2D material also depends on the interlayer coupling and layers number. Figure ubstrate , 37. It 2 hetero-structure were significantly different from that of exposed graphene region, as shown in Fig. 2, which confirms the presence of graphene/WS2 hetero-structure. Next, WS2 material can suppress the 2D characteristic peak intensity of graphene. Finally, both G peak and 2D peak of graphene/WS2 hetero-structure shift upward compared with the spectra of bare graphene material. Due to the interlayer coupling between graphene and WS2, the 2D peak would also shift up, and the mechanical strain also has the impact on the Raman shift of graphene. The enhancement factor (EF) is the ratio of the maximum peak intensity of graphene/WS2 hetero-structure divided by the maximum peak intensity of graphene. The maximum peak intensity of G peak increases from 460 to 830, and the maximum peak intensity of 2D peak increases from 340 to 1460, and the corresponding EF were 1.8 and 4.3, respectively. The D peak signal is significantly enhanced when the graphene/TMDs hetero-structures is formed. Therefore, the ID/IG ratio of monolayer graphene is weaker than that of graphene/WS2 hetero-structure. This is because the extrusion of WS2 on graphene has the effect on the structure of graphene, which would result in the appearance of a small number of defects. In Fig. 2 hetero-structure is higher than that of bare graphene, which may be related to the effective interlayer coupling and the strain effect. Meanwhile, the maximum intensity of PL spectra is increased from 270 to 1410, and the corresponding EF is 5.23. The intensity enhancement of characteristic peak can be attributed to the coupling of graphene/WS2 hetero-structure.The Raman spectra of graphene/WS2 and graphene/WS2 hetero-structure is shown in Fig. 2, the A1g mode characteristic peak position of graphene/WS2 hetero-structure was blue-shifted, and the intensity of E12g mode and A1g mode characteristic peaks was higher than those of WS2, and the graphene/WS2 hetero-structure film has the excellent crystallinity. The reason is that the coupling between layers can be enhanced when the two materials are stacked to form the hetero-structure, which would generate the interlayer interaction forces. The maximum E12g and Alg characteristic peak intensity increases from 3400 and 1100 to 6500 and 2950, respectively. And the enhancement factors (EF) are 1.9 and 2.7, respectively. In addition, monolayer WS2 and multilayer WS2 are the direct band gap semiconductor and indirect semiconductor materials, respectively. Therefore, the PL spectroscopy can be used to identify the layer number of WS2 sample. In Fig. 2. The PL intensity of graphene/WS2 hetero-structure was stronger than that of monolayer WS2. The reasons are the following: First, the work function between graphene and WS2 does not match. Second, the internal field was formed. Third, the photoelectrons from WS2 can transfer to graphene. Forth, the WS2 material retains holes. The maximum intensity of strongest peak increases from 7450 to 19,320, and the EF of PL spectra are 2.6. The increase in peak intensity is due to the coupling between graphene and WS2 materials.Raman spectroscopy can be used to evaluate the crystal quality and film thickness of 2D materials. The Raman spectra comparison of WS2 hetero-structure on SiO2/Si substrate are shown in Fig. 2/Si substrate was not much different from the original one. The surface was relatively clean except for a few particles in some areas. These results indicate that the graphene film is uniformly and completely formed. The MoS2 thin film covers graphene/SiO2/Si substrate, which can be connected into the continuous graphene thin film across the grain boundaries. The prepared graphene/MoS2 hetero-structure was continuous and intact, and the sample surface was relatively clean, which has the good surface uniformity. The local fluorescence intensity distribution is not uniform when there are many defects. Figure 2 film. The crystal lattice of sample has the fewer defects. In Fig. 2/Si substrate is measured to judge the material thickness, the thickness of monolayer MoS2 material is about 0.81 nm. It can be found by the SEM test result that the morphology of MoS2 film sample is the triangular flake, as shown in Fig. 2 hetero-structure has been successfully prepared.The optical microscope pictures of graphene/MoS1g mode characteristic peak is red-shifted, whereas the characteristic peak of E12g mode is blue-shifted. As a result, the frequency distance between A1g and E12g vibration modes becomes smaller, which can be used to identify the thickness of 2D materials. Figure 2 at different positions. The characteristic peaks of E12g mode and A1g mode were at 381.2 cm−1 and 400.5 cm−1, respectively. And the peak spacing was 19.3 cm−1, which indicates the presence of monolayer MoS2. Due to the Van der Waals force between the layers, the frequencies of two vibration modes moving in the same or opposite directions between adjacent atoms in the layers are slightly different. The PL spectra are used to obtain the light emission characteristics of MoS2 film, as shown in Fig. 2 was much greater than that of multilayer, and the electronic band structure changed from indirect band gap to direct band gap when the layer number of MoS2 material changed from multilayer to single layer. Therefore, there was only the strong emission peak of monolayer MoS2. In addition, the strongest PL peak was at 678.5 nm, and the corresponding direct band gap was 1.83 eV, which is close to the band gap value of mechanically peeling MoS2 film. It can be found by observing Fig. 12g and A1g mode characteristic peak were blue-shifted. This is because the Raman peak line would have a certain frequency shift with increase in the temperature and laser power. Figure 2, the luminous intensity increasing accordingly with increase in the laser power, and the strongest PL peak position was blue-shifted.The interlayer interaction weakens with decrease in the film thickness. The A2 hetero-structure, and there were the G, 2D peaks of graphene and the E12g and A1g peaks of MoS2, which indicates the formation of layered graphene/MoS2 hetero-structure material. The E12g and A1g Raman characteristic peaks of MoS2 were located at 375.5 cm−1 and 394.4 cm−1, respectively. And the peak spacing was 18.9 cm−1. Compared with intrinsic graphene, the G peak and 2D peak positions of graphene/MoS2 hetero-structure shift to large wavenumbers, and G peak and 2D peak move from 1581 and 2672 cm−1 to 1587 and 2674 cm−1, respectively. In addition, the intensity of G peak is stronger than that of 2D peak. The rise of the 2D and G peaks position is related to the effective interlayer coupling and the strain effect. Compared with the Raman spectra of MoS2 material, the spectra of graphene/MoS2 hetero-structure material are significantly shifted due to the enhancement of interlayer atomic interaction, and the peak intensity can also be significantly enhanced. It can be found from Fig. 2 hetero-structure has two absorption peaks at 621 nm and 683 nm and that the corresponding band gaps were 1.99 eV and 1.82 eV according to the conversion relationship between wavelength and electron volt. The luminous intensity of graphene/MoS2 hetero-structure was lower than that of intrinsic MoS2. The reasons of these phenomena are that the graphene material has the weakening effect on the fluorescence of MoS2 material and that the electronic energy band and electronic distribution can be changed due to the interlayer coupling, which can greatly change the PL and Raman spectra.A high-frequency layer vibrating phonon mode of monolayer 2D material would split into the N corresponding high-frequency modes in an N-layer 2D material, which would lead to the Davydov splitting. Figure 2 hetero-structure, the Raman peaks intensity of G, 2D, E12g, and A1g increasing with increase in the laser power. The peak position difference between E12g and A1g is gradually enhanced with increase in the layer number of MoS2 material. The characteristic peak positions of E12g and A1g were 377.2 cm−1 and 396.7 cm−1, respectively. And the peak position difference was 19.5 cm−1, which can be judged that MoS2 material is the monolayer. Meanwhile, the G and 2D peaks of graphene were red-shifted and blue-shifted, respectively. This is because graphene material is doped with MoS2. It can be found by observing Fig. 2 hetero-structure. These PL peak corresponding to the compound transition of A and B excitons, wherein the light emission corresponding to the direct band gap exciton recombination was 1.84 eV, whereas the peak corresponding to the indirect band gap exciton recombination was at 2.0 eV. The luminous intensity of strongest peak is increased with increase in the laser power, and the peak position of the strongest PL spectra is red-shifted. This is due to the p-type conductivity of the graphene and the change of band structure when graphene and MoS2 materials were stacked. In addition, the arrangement of energy bands at the interface allows the electrons from electron-rich MoS2 to transfer to p-type graphene material.Figure 2 and MoS2 were used as TMDs material, were successfully synthesized directly on graphene films by using APCVD. The morphology, spectral characteristics and luminescence law of hetero-structures can be obtained by AFM, SEM, EDX, Raman and PL spectroscopy, and the hetero-structures show the excellent photosensitivity. Compared with intrinsic graphene material, the G and 2D peak positions of graphene/TMDs hetero-structures are the blue-shifted, the intensity of G peak is stronger than that of 2D peak with increase in the laser power and decrease in the I2D/IG ratio. Due to the presence of internal electric field, the photo-generated electron–hole pairs can be effectively separated at the interface of graphene/TMDs hetero-structures, which could greatly improve the light response. This research could effectively guide the preparation process improvement in large-area, high-quality hetero-structures, and it could also pave the way for the application of graphene/TMDs hetero-structures in the optoelectronic devices field.Graphene/TMDs-based hetero-structures, where WS


Background: Many medical schools train their faculty members to construct high cognitive level multiple choice questions (MCQs) that demand a great deal of analytical and critical thinking, application, and competence. The purpose of this study is to determine the cognitive levels of MCQs by using Moore’s Expanded Outcomes Framework and to understand whether the quality of MCQs has an effect on students’ assessment performance.Methods: Four trained faculty members analysed 100 randomly selected questions developed at the University Clinic of Dentistry (UCD) and 100 questions developed by the National Board of Dental Examinations (NBDE). Moore’s framework was applied to assist the review process.Results: The majority of questions was at the level of declarative knowledge followed by questions at the level of procedural knowledge. The cognitive level of UCD questions from 2002 to 2009 was significantly lower than that of NBDE questions but increased in questions written from 2010 to 2018. The improvement of quality of MCQs had no impact on assessment performance of students.Conclusion: The enhanced cognitive levels of UCD MCQs written 2010–2018 coincides with the implementation of a faculty training program for writing high-ordered MCQs. In addition, this study shows that the use of Moore’s expanded framework is on par with other known taxonomies in supporting educators in writing items and reviewing the process.Abbreviations: MCQs: Multiple Choice Questions; UCD: University Clinic of Dentistry; NBDE: National Board of Dental Examinations MCQs are commonly used in assessments at undergraduate and postgraduate levels of medical examinations due to the fact that they are time-efficient, highly reliable and conveniently standardized ,2. Well-There are numerous frameworks to support the educators’ understanding of the development of learning associated with educational programs. These programs include developmental frameworks and analytical models that address knowledge, skills and attitudes. Some frameworks such as Blooms’ Taxonomy and Millers’ Pyramid are well known and have been used in undergraduate education for creating multiple choice type questions ,6,7. TheThe purpose of this study was to analyse the cognitive levels of MCQs used for summative assessment at the end of the fourth year in a clinical undergraduate curriculum at the University Clinic of Dentistry (UCD) at the Medical University of Vienna by using Moore’s expanded outcomes framework because of its accordance with well-known taxonomies . The cogAdditionally, we analysed NBDE Part II questions for evaluating the comparability or similarity of cognitive levels of our MCQs. NBDE is conducted in the USA for determining qualifications of dentists who seek licensure to practice dentistry. It assesses the ability to understand important information from basic biomedical and dental sciences as well as the ability to apply such information in a problem-solving context . With reth year consists of 6 modules and covers areas as paediatric dentistry, cariology, dental restoration, endodontics, periodontology, prophylaxis, restorative dentistry, permanent dentures, prosthetics, surgery and orthodontic followed by a clinical internship of 72 weeks in the last 5th and 6th academic years that focuses on intensive clinical training [The current dentistry program at the Medical University of Vienna was introduced in 2002, together with the newly integrated curriculum for medical students in the so-called Vienna Medical Curriculum (VCM) (6). VCM of Dentistry degree programme is based on integration of non-clinical and clinical learning: in each learning unit (Module) students learn about structure and function, as well as the most important and most common diseases and therapies. In addition, there are courses called ‘Lines’ which take place throughout the semester. The Lines connect module content with clinical work by focusing on clinical skills. Problem-based learning (PBL) and case-based learning (CBL) supplement other instructional methods. The duration of the Dentistry degree programme is 12 semesters: The first two academic years of this program is partially identical with human medicine and focuses on basic medical sciences. The third year of study is composed of dental-oriented courses and covers three modules such as chewing and locomotor system, oral pathology and internal organs as well as brain, muscles and sensory organs. The dentistry degree programme in 4training .th year and ends at the end of 6th year with oral examination.Since 2002, a range of different assessment formats are used. Written examinations such as multiple choice questions has been implemented to curriculum to enhance the objectivity and reliability. Continuous assessments in written format or oral contribution take place at the end of each practical training and seminars. Some seminars follow a team-based-learning format. The formative-integrated examinations in MCQs format take place at the end of each winter semester to provide students with feedback on their learning performance. The summative-integrated examination in MCQ format takes place at the end of each academic year. The last summative-integrated examination in MCQ format is integrated at the end of fourth year which contains in total 165 MCQs and is assessing students’ competence and clinical knowledge in subject areas and materials from an entire fourth academic year before entering the dental practice environment which starts in the 5Starting in 2001, the quality of MCQs is evaluated by an interdisciplinary review committee that consists of specialized experts, exam auditors and module coordinators. The committee’s tasks are correcting and formatting and of grammatical errors and detecting ambiguity in the interpretation of questions. Items deemed ambiguous, flawed or irrelevant to the committee were returned to the authoring faculty member with suggestions for improvement. Only questions that were approved by the committee are added into our item bank. With regard to quality assurance of MCQs, training programs are necessary to construct high quality MCQs. Since 2010, a mandatory training program, called ‘Examining clinical competence of medical students with multiple-choice questions’, has been integrated in advanced medical education courses for junior faculty members. Currently, this training programme is also referring to faculty tenure as an optional workshop to qualify for permanent position. Our MCQs are written by both trained and non-trained faculty members using Millers’ pyramid and internal quality criteria such as reliability, validity and discrimination index. Point biserial correlation is used to determine the discrimination index of items. The content validity of assessment has been increased by developing blueprint process for each summative-integrated assessments. MCQs are constructed to test knowledge and definitions of facts (‘knows’), but also require reaching a conclusion, formulating a prognosis, or selecting options for action (‘knows how’).The objectives of this study were (1) to determine whether MCQs from UCD have an international comparable cognitive level (UCD vs. NBDE); (2) to address the development of MCQs from 2002 to 2018; (3) to analyse the impact of different subject areas on constructing high ordered MCQs that promote and measure competency and critical thinking ability of students; (4) to evaluate the role of item difficulty – an important parameter for increasing the MCQs’ quality – on the distribution of cognitive level of our MCQs.In total, two hundred MCQs  were evaluated. For setting up MCQs pools from our item bank for the fourth year summative-integrated examination, which contains 4933 approved MCQs, we used the following criteria: (1) MCQs that had been used only one time; (2) 50% of MCQs dated from 2002 to 2009 (pool 1) and 50% from 2010 to 2018 (pool 2) to evaluate the distribution of the cognitive levels of MCQs over the course of time.Fifty UCD-MCQs from each pool were selected using a random number generator. Using these particular questions, we wanted to determine if we are able to develop high levels of MCQs for our students to further examine their critical and analytical thinking skills.100 of the 200 NBDE Part II questions were chosen randomly from the officially released item test package dating approximately from 2000 to 2008. A request to the American Dental Association for questions from 2009 to 2018 for study purposes was not honoured, and the questions have not been provided. Therefore, we were just able to analyse 100 MCQs of NBDE and compared with our 50 MCQs from year 2002 to 2009.All reviewers, who participated this study, had participated in faculty development program directed towards the construction of MCQs. The two faculty members with clinical experience in dentistry assessed and interpreted the questions at the clinical level. They have both had several years’ experience on constructing of MCQs. Another faculty member was a regular item writer with research experience in the field of theoretical dentistry. The last reviewer had several years of experience in preparing exams and selecting exam questions in the dental field.MCQs were analysed by four faculty members with expertise in different fields. The reviewers received 25 questions together with the coding form  once eveSample size was calculated by performing a power analysis with power of 0.8, a significance level of 0.05 for a medium effect size according to Cohen . All dat2 = 9.234; p = 0.005) between the distribution of cognitive levels of NBDE questions and UCD questions  were analysed. Of 50 MCQs from UCD, 94% questions were categorized as Level 3a, and 6% as Level 3b. On the other hand, the distribution of MCQs from NBDE was as follows: 73% Level 3a, 26% Level 3b and 1% Level 4. According to this data, there was a significant difference .Fifty MCQs from the years 2002 to 2009 and 50 MCQs from the years 2010 to 2018 were analysed . The oldThe results showed that the quality of MCQs had no impact on students’ exam performance. The grade average was 3.3 from 2002 to 2009 and from 2010 to 2018.2 = 13.021; p = 0.188).To understand the role of specific fields on the distribution of cognitive levels of MCQs, the questions were analysed in terms of their disciplines. The data showed more than 50% of recall questions in all fields. Similar distribution of knowledge questions (59%) and application questions (41%) was seen in the field of prosthetic fundaments and removable prosthetic. In the field of cardiology and orthodontics, the number of application-oriented MCQs increased to up to 22%. The analysis also showed 94% knowledge questions in the field of dental surgery . Only on2 = 7.278; p = 0.534), and the data were not able to show the impact of difficulty of items on the distribution of the cognitive level of MCQs  is one of the most commonly used statistic parameter for item analysis, and is obtained by dividing the number of students who answered the item correctly by the total number of students who answered that item, thus ranging between 0.0 and 1.0 –19. 82%  of MCQs .Table 6.In this study, we evaluated the cognitive levels of MCQs used for summative assessment after the fourth year in a clinical undergraduate curriculum at the UCD by using Moore’s expanded outcomes framework. We analysed whether our currently used MCQs were assessing our students’ abilities at the procedural knowledge level and competence level rather than just at the level of declarative knowledge. We demonstrated that the construction of MCQs at the UCD has improved over time and the vast majority of knowledge questions have been replaced by items measuring procedural knowledge of undergraduate dental students.The reasons for this could be first, the amount of MCQs promoting longer-term retention of knowledge has increased since a training program has been implemented into advance educational activities for junior faculty members. Second, some faculty members who developing high quality questions may be more interested in constructing questions that measure the application of knowledge and clinical competence of examinees than others. The third reason could be that crafting declarative knowledge questions has already been used up by our item writers therefore they now constructing MCQs that explore a deeper understanding of basic science content which demand the use of higher-order thinking skills.Previous studies have indicated, that low cognitive level MCQs are mostly written by untrained item writers and constructing MCQs is a creative act that can be mastered through extensive and critically supervised practice ,21. AfteOn the other hand, writing MCQs with high cognitive levels requires a multistep preparation process. An interdisciplinary review committee is one of the critical steps in construction and writing of items. Wallach et al. demonstrated that reading and discussing questions in a committee review process improves not just the quality of questions but also increases the knowledge and information of item writers between various disciplines . As the Nevertheless, it is also important to ask if it is necessary to assess medical students’ knowledge at the competence level by using MCQ format. Several other methods have been developed and implemented over the time that have focused on clinical skills like taking a history from a patient and performing a physical examination, communication skills, procedural skills, and professionalism such as OSCE. In Miller’s model, written examinations measure what student ‘knows’ or ‘knows how’, and OSCE is a tool for assessing ‘shows how’ . In MoorOne limitation of our study is the small number of MCQs that were analysed, which also effects on evaluating the power of ‘difficulty of items and specific fields’ on cognitive levels. Another limitation is the lack of availability of officially published NBDE questions from 2009 to 2018. Therefore, we were not able to compare our newly written MCQs which might have similar amount of high level questions. In spite of the abovementioned limitations, our study results show that training and internal review process enhance the development of higher quality multiple-choice questions that align with the Moore’s framework of Level 3b and Level 4.Written examinations are capable of assessing application of knowledge and clinical competence. We observe that the items that require the student to think analytically and to apply their knowledge to practically relevant problems have been increased over time. In order to successfully enhance the quality of MCQs to reach the higher international level, we highly recommend the implementation of Moore’s expanded framework for writing and reviewing items as a first step.