The luminescence mechanism of ligand-induced interface states in silicon quantum dots.

Over decades of research on photoluminescence (PL) of silicon quantum dots (Si-QDs), extensive exploratory experiments have been conducted to find ways to improve the photoluminescence quantum yield. However, the complete physical picture of Si-QD luminescence is not yet clear and needs to be studied in depth. In this work, which considers the quantum size effect and surface effect, the optical properties of Si-QDs with different sizes and surface terminated ligands were calculated based on first principles calculations. The results show that there are significant differences in the emission wavelength and emission intensity of Si-QD interface states connected by different ligands, among which the emission of silicon-oxygen double bonds is the strongest. When the size of the Si-QD increases, the influence of the surface effect weakens, and only the silicon-oxygen double bonds still localize the charge near the ligand, maintaining a high-intensity luminescence. In addition, the presence of surface dangling bonds also affects luminescence. This study deepens the understanding of the photoluminescence mechanism of Si-QDs, and provides a direction for both future improvement of the photoluminescence quantum efficiency of silicon nanocrystals and for fabricating silicon-based photonic devices.

A Silicon Sub-Bandgap Near-Infrared Photodetector with High Detectivity Based on Textured Si/Au Nanoparticle Schottky Junctions Covered with Graphene Film.

We present a straightforward approach to develop a high-detectivity silicon (Si) sub-bandgap near-infrared (NIR) photodetector (PD) based on textured Si/Au nanoparticle (NP) Schottky junctions coated with graphene film. This is a photovoltaic-type PD that operates at 0 V bias. The texturing of Si is to trap light for NIR absorption enhancement, and Schottky junctions facilitate sub-bandgap NIR absorption and internal photoemission. Both Au NPs and the texturing of Si were made in self-organized processes. Graphene offers additional pathways for hot electron transport and to increase photocurrent. Under 1319 nm illumination at room temperature, a responsivity of 3.9 mA/W and detectivity of 7.2 × 1010 cm × (Hz)1/2/W were obtained. Additionally, at -60 °C, the detectivity increased to 1.5 × 1011 cm × (Hz)1/2/W, with the dark current density reduced and responsivity unchanged. The result of this work demonstrates a facile method to create high-performance Si sub-bandgap NIR PDs for promising applications at ambient temperatures.

Proactive and reactive language control in bilingual language production revealed by decoding sustained potentials and electroencephalography oscillations.

Adopting highly sensitive multivariate electroencephalography (EEG) and alpha-band decoding analyses, the present study investigated proactive and reactive language control during bilingual language production. In a language-switching task, Chinese-English bilinguals were asked to name pictures based on visually presented cues. EEG and alpha-band decoding accuracy associated with switch and non-switch trials were used as indicators for inhibition over the non-target language. Multivariate EEG decoding analyses showed that the decoding accuracy in L1 but not in L2, was above chance level shortly after cue onset. In addition, alpha-band decoding results showed that the decoding accuracy in L1 rose above chance level in an early time window and a late time window locked to the stimulus. Together, these asymmetric patterns of decoding accuracy indicate that both proactive and reactive attentional control over the dominant L1 are exerted during bilingual word production, with a possibility of overlap between two control mechanisms. We addressed theoretical implications based on these findings for bilingual language control models.

Processing negative emotion in two languages of bilinguals: Accommodation and assimilation of the neural pathways based on a meta-analysis.

Numerous functional magnetic resonance imaging (fMRI) studies have examined the neural mechanisms of negative emotional words, but scarce evidence is available for the interactions among related brain regions from the functional brain connectivity perspective. Moreover, few studies have addressed the neural networks for negative word processing in bilinguals. To fill this gap, the current study examined the brain networks for processing negative words in the first language (L1) and the second language (L2) with Chinese-English bilinguals. To identify objective indicators associated with negative word processing, we first conducted a coordinate-based meta-analysis on contrasts between negative and neutral words (including 32 contrasts from 1589 participants) using the activation likelihood estimation method. Results showed that the left medial prefrontal cortex (mPFC), the left inferior frontal gyrus (IFG), the left posterior cingulate cortex (PCC), the left amygdala, the left inferior temporal gyrus (ITG), and the left thalamus were involved in processing negative words. Next, these six clusters were used as regions of interest in effective connectivity analyses using extended unified structural equation modeling to pinpoint the brain networks for bilingual negative word processing. Brain network results revealed two pathways for negative word processing in L1: a dorsal pathway consisting of the left IFG, the left mPFC, and the left PCC, and a ventral pathway involving the left amygdala, the left ITG, and the left thalamus. We further investigated the similarity and difference between brain networks for negative word processing in L1 and L2. The findings revealed similarities in the dorsal pathway, as well as differences primarily in the ventral pathway, indicating both neural assimilation and accommodation across processing negative emotion in two languages of bilinguals.

Sub-bandgap near-infrared photovoltaic response in Au/Al2O3/n-Si metal-insulator-semiconductor structure by plasmon-enhanced internal photoemission.

Silicon sub-bandgap near-infrared (NIR) (λ > 1100 nm) photovoltaic (PV) response by plasmon-enhanced internal photoemission was investigated. The Si sub-bandgap NIR PV response, which remains unexploited in Schottky junction-like solar cell device, was examined using nanometer sized Au/Al2O3/n-Si junction arrays. This kind of metal-insulator-semiconductor structure was similar in functionality to Schottky junction in NIR absorption, photo-induced charge separation and collection. It showed that NIR absorption increased steadily with increasing volume of Au nanoparticles (NPs) till a saturation was reached. Simulation results indicated the formation of localized surface plasmon on the surfaces of Au NPs, which was correlated well with the observed NIR absorption. On the other hand, the NIR PV response was found sensitive to the amount and size of Au NPs and thickness of Al2O3. Chemical and field-effect passivation of n-Si by using Al2O3 and SiO2 were used to optimize the NIR PV response. In the current configuration, the best PV conversion efficiency was 0.034% at λ = 1319 nm under illumination power of 0.1 W/cm2.

Language switching training modulates the neural network of non-linguistic cognitive control.

Bilingual language experience, such as switching between languages, has been shown to shape both cognitive and neural mechanisms of non-linguistic cognitive control. However, the neural adaptations induced by language switching remain unclear. Using fMRI, the current study examined the impact of short-term language switching training on the neural network of domain-general cognitive control for unbalanced Chinese-English bilinguals. Effective connectivity maps were constructed by using the extended unified structural equation models (euSEM) within 10 common brain regions involved in both language control and domain-general cognitive control. Results showed that, the dorsal anterior cingulate cortex/pre-supplementary motor area (dACC/pre-SMA) lost connection from the right thalamus after training, suggesting that less neural connectivity was required to complete the same domain-general cognitive control task. These findings not only provide direct evidence for the modulation of language switching training on the neural interaction of domain-general cognitive control, but also have important implications for revealing the potential neurocognitive adaptation effects of specific bilingual language experiences.

Neural interaction between language control and cognitive control: Evidence from cross-task adaptation.

It has been documented that conflict adaptation (conflict resolution in a task enhanced by that in a previous task) exists not only in the same domain but also across different domains with shared cognitive control mechanisms. For the first time, the present study adopted a cross-task adaptive blocked design to examine the relationship between bilingual language control and cognitive control from the perspective of the immediately adjacent, mutual influence on the neural connectivity level. The results showed that the conflict setting induced by previous tasks changed the nodal degrees of the anterior cingulate cortex/presupplementary motor area and the right thalamus, and connectivity strength of shared links between adjacent language and cognitive control tasks. In addition, pre-activation of the cognitive control network affected the transitivity of the successive use of the language control network. These findings not only indicate a cross-task adaptation effect on the neural connectivity level, but also provide evidence for similarities in conflict detection and inhibition control between language-specific control and domain-general cognitive control. In addition, our results also suggest that there is only partial overlap between bilingual language control and domain-general cognitive control.

Speech perception in a second language: Hearing is believing-seeing is not.

This study examined the effects of the speaker's face and accent on second language (L2) speech perception. Forty-two Chinese speakers of English immersed in the L2 environment were instructed to perform a cross-modal semantic judgement task. They saw an Asian or Caucasian face on the screen and heard word pairs in L2 in a native English accent or a Chinese accent, and were asked to judge whether the word pairs were related to each other in meaning or not. Results showed that for words presented in the native accent, there was a semantic effect in both reaction time and accuracy, irrespective of the face shown. For words presented in the non-native accent, the RT data showed a semantic effect, whereas the accuracy revealed a reversed semantic effect. The speed-accuracy trade-off suggests a relatively weak semantic effect. These patterns were not modulated by the faces accompanying the word pairs. These results suggest that the cue of accent plays an important role during bilinguals' speech perception in L2, such that non-native accent hampers speech perception, even when it matches bilinguals' first language. In contrast, bilinguals do not seem to depend on the social indexical cue of the face when it is not reliable. The present findings hold implications for the Bilingual Model of Lexical Access (BIMOLA) of bilingual speech perception and the monolingual models of social speech perception.

The influence of short-term language-switching training on the plasticity of the cognitive control mechanism in bilingual word production.

This study examined the effect of short-term language-switching training on the cognitive control mechanism in bilingual word production. In two experiments, two groups of relatively proficient but unbalanced Chinese-English bilinguals performed a cued picture-naming task, in which they switched between their two languages. On two consecutive days, the participants took part in four sessions. The same procedure was employed on 2 days in Experiment 1, whereas the cue-language mapping was reversed on Day 2 in Experiment 2. In both experiments, picture naming in the dominant language (L1, Chinese) was slower than that in the weaker second language (L2, English) in all sessions. In addition, the reversed language dominance effect was enhanced with training, suggesting that training proactively increases the amount of inhibition of the dominant L1 at the global level. Furthermore, switching costs in the L1 were reduced with training in Experiment 1, but not in Experiment 2. These results indicate that language-switching training improves the efficiency of reactively exerting inhibitory control over the dominant L1 at the local level. However, when a cue matches with different target languages, the effect of training is absent at the local level. These findings reveal the plasticity and complexity of the cognitive control mechanism as a function of bilingual experience, particularly in language switching.

Chinese Learners of English See Chinese Words When Reading English Words.

The present study examines when second language (L2) learners read words in the L2, whether the orthography and/or phonology of the translation words in the first language (L1) is activated and whether the patterns would be modulated by the proficiency in the L2. In two experiments, two groups of Chinese learners of English immersed in the L1 environment, one less proficient and the other more proficient in English, performed a translation recognition task. In this task, participants judged whether pairs of words, with an L2 word preceding an L1 word, were translation words or not. The critical conditions compared the performance of learners to reject distractors that were related to the translation word (e.g., , pronounced as /bei 1/) of an L2 word (e.g., cup) in orthography (e.g., , bad in Chinese, pronounced as /huai 4/) or phonology (e.g., , sad in Chinese, pronounced as /bei 1/). Results of Experiment 1 showed less proficient learners were slower and less accurate to reject translation orthography distractors, as compared to unrelated controls, demonstrating a robust translation orthography interference effect. In contrast, their performance was not significantly different when rejecting translation phonology distractors, relative to unrelated controls, showing no translation phonology interference. The same patterns were observed in more proficient learners in Experiment 2. Together, these results suggest that when Chinese learners of English read English words, the orthographic information, but not the phonological information of the Chinese translation words is activated. In addition, this activation is not modulated by L2 proficiency.

Neural correlates for naming disadvantage of the dominant language in bilingual word production.

The present study investigated the neural correlates of naming disadvantage of the dominant language under the mixed language context. Twenty one unbalanced Chinese-English bilinguals completed a cued picture naming task while being scanned with functional magnetic resonance imaging (fMRI). Behavioral results showed that naming pictures in the second lanuage (L2) was significantly slower than naming pictures in the first language (L1) under a single language context. When comparing picture naming in L2 to naming in L1, enhanced activity in the left inferior parietal lobule and left cerebellum was observed. On the contrary, naming pictures in Chinese (L1) was significantly slower than naming in English (L2) under the mixed language context. The fMRI results showed that bilateral inferior frontal gyri, right middle frontal gyrus, and right supplementary motor area were activated to a greater extent in L1 than in L2. These results suggest that the dominant language is inhibited to a greater extent to ensure the production of the second language under the mixed language context. Therefore, more attentional control resources are recruited when bilinguals produced the dominant language. The present study, for the first time, reveals neural correlates of L1 naming disadvantage under the mixed language context.

Short-term language switching training tunes the neural correlates of cognitive control in bilingual language production.

The present study investigated how language switching experience would modulate the neural correlates of cognitive control involved in bilingual language production. A group of unbalanced Chinese-English bilinguals undertook an 8-day cued picture naming training during which they named pictures in either of their languages based on visually presented cues. Participants' brain activation was scanned before and after the training in the same task. Behavioral results revealed a significant training effect such that switch costs were reduced after training. fMRI results showed that after training, activation of brain areas associated with cognitive control including the anterior cingulated cortex and the caudate was reduced. Besides, the activation reduction in the left dorsal anterior cingulated cortex positively correlated with the reduction in switch costs in response time and this training effect could be transferred to untrained stimuli. These findings suggest that neural correlates of cognitive control, especially that of the conflict monitoring process, in bilingual language production could be modulated by short-term language switching training. Hum Brain Mapp 38:5859-5870, 2017. © 2017 Wiley Periodicals, Inc.

Improving proactive control with training on language switching in bilinguals.

The present study examined how a short period of language switching training affects nonverbal cognitive control, as measured by the AX version of the Continuous Performance Test. A group of Chinese-English bilinguals were trained over 10 days on a picture naming task that required switching between languages. We recorded their behavioral performance and event-related potentials before and after the training to examine its effects on cognitive performance. The behavioral measurement of proactive control, that is, goal maintenance before the occurrence of the target, is significantly larger in the post-training phase as compared with the pretraining phase, indicating a proactive control shift. The event-related potential results show that the training led to an increase in the mean amplitude of the N2 component, elicited by both the cue and the probe stimuli. A group of control participants who did not undergo training showed an enlarged N2 only for the probe but not for the cue stimuli in the second as compared with the first phase of testing. No variations in behavioral performance were found in the control group between the two phases of testing. These findings suggest that language switching training enhances proactive control in bilinguals.

An ERP study of inhibition of non-target languages in trilingual word production.

The present study examined the locus where inhibition of non-target languages is exerted during trilingual word production by analyzing the cue-locked and stimulus-locked ERPs respectively in the n-2 language repetition paradigm. During the experiment, Uighur-Chinese-English trilinguals overtly named Arabic digits in one of their three languages according to a visually presented cue while their behavioral and electrophysiological responses were recorded. The behavioral data revealed insignificant n-2 repetition costs. Cue-locked ERPs revealed also only tiny or marginally significant n-2 repetition effects over some midline electrodes. The stimulus-locked ERP data showed a more negative ERP component elicited by the n-2 repetition trials than the n-2 non-repetition trials around 250 ms after stimulus onset, but no significant difference in this ERP effect between different languages was found. The results indicate that inhibition of non-target languages occurs at the lemma selection phase rather than the language task schemas phase during trilingual language production.