High Photoresponsivity and Short Photoresponse Times in Few-Layered WSe₂ Transistors

Here, we report the photoconducting response of field-effect transistors based on three atomic layers of chemical vapor transport grown WSe₂ crystals mechanically exfoliated onto SiO₂. We find that trilayered WSe₂ field-effect transistors, built with the simplest possible architecture, can display high hole mobilities ranging from 350 cm²/(V s) at room temperature (saturating at a value of ∼500 cm²/(V s) below 50 K) displaying a strong photocurrent response, which leads to exceptionally high photoresponsivities up to 7 A/W under white light illumination of the entire channel for power densities <i>p</i> < 10² W/m². Under a fixed wavelength of λ = 532 nm and a laser spot size smaller than the conducting channel area, we extract photoresponsitivities approaching 100 mA/W with concomitantly high external quantum efficiencies up to ∼40% at room temperature. These values surpass values recently reported from more complex architectures, such as graphene and transition metal dichalcogenides based heterostructures. Also, trilayered WSe₂ phototransistors display photoresponse times on the order of 10 μs. Our results indicate that the addition of a few atomic layers considerably decreases the photoresponse times, probably by minimizing the interaction with the substrates, while maintaining a very high photoresponsivity

Within-subject classification accuracies.

<p>Means and 95% confidence intervals for within-subject classification of seen actions in different regions of interest (ROIs). Dashed line indicates the chance level.</p

Brain regions showing activation during action execution and observation localizers.

<p>For each subject, the statistical image was thresholded at T > 2 and binarized. Warm colors indicate number of subjects that showed activation during action observation for each voxel, and cold colors during action execution. Purple color shows overlap between action execution and observation. Abbreviations: SPL–Superior Parietal Lobule; SOP–Superior Occipital Pole; SMG–Supramarginal Gyrus; LOC–Lateral Occipital Cortex, SI–primary somatosensory cortex, SII–secondary somatosensory cortex.</p

Schematic description of data preprocessing and analysis for hyperclassification.

<p>Bayesian canonical correlation was used on preprocessed data to acquire mapping between actor’s and observer’s BOLD signals. Mapping was acquired in cross-validated fashion, where a model was trained on four runs of the actor and the observer. The observer’s left-out run was used in subsequent analysis, where shared representation between actor and observer was mapped to actor’s functional space and used in testing the classifier. Bayesian logistic regression was used as pattern classifier. In within-subject classification training and testing was done using the data from the same individual. In hyperclassification training was done on actor’s data and testing on corresponding observer’s data.</p

MNI coordinates of clusters included in the overlap localizer ROI.

<p>The individual subject data obtained in action-execution and action-observation localizers were thresholded at <i>T</i> > 2, and supra-threshold voxels overlapping in both execution and observation were preserved. Labels provided from Harvard-Oxford cortical and subcortical structural atlas (FSL).</p

Confusion matrix for functionally realigned hyperclassification with meta-analytic ROI.

<p>Numbers in the cells indicate percentage of action category guesses by classifier. Black frames denote action categories that were correctly classified with above chance level accuracy.</p

Action-specific brain activation.

<p>Brain regions whose activity increased during specific actions (A: Power grip, B: Precision grip, C: Slap, D: Point) during the main experiment. Warm colors indicate activation for observer subjects, cold colors activation for actor subjects, and purple color overlap between the activation maps of actors and observers.</p

Brain activation during closed-eyes and open-eyes experiments.

<p>Brain regions showing significant activation during action execution and observation in the main experiment for closed-eyes (top) and open-eyes (bottom) conditions for actor and all observers. Warm colors indicate activation for observer subjects; cold colors indicate activation for actor subjects. Purple color indicates overlap between activation maps of actors and observers.</p

MNI coordinates of clusters included in the meta-analytic ROI.

<p>Labels provided from Harvard-Oxford cortical and subcortical structural atlas (FSL).</p

Experimental design and sample trials of action execution and observation.

<p>A) The actor performed four different actions in the scanner. An actor in the control experiment kept her eyes closed and could not see her own hand actions. B) Trial structure for the actor subjects. C) Trial structure for the observer subjects. Green outline shows the trial portions used for classification. Photographer identified themselves and the purpose of the photograph to the person shown in the photograph, and they agreed to have their photograph taken and potentially published.</p