Photoelectrical Response in Single‐Layer Graphene Transistors

The illumination of single‐layer graphene (SLG) transistors with visible light causes a negative shift in their transfer curves, attributable to the desorption of oxygen. However, their hysteresis is not affected by illumination, which suggests that charge traps are not affected by the visible‐light exposure. When SLG transistors are covered with a layer of photoactive polymer, the photodesorption‐induced current change in the transistors becomes less significant than the effects caused by the surrounding photoactive polymer. These observations demonstrate that the photoelectrical response of SLG transistors is dominated by extrinsic mechanisms rather than by the direct photocurrent process. The results suggest a new strategy for achieving light detection. The large cross section of SLG films for receiving photons and the capability of tailoring photoelectrical properties on them is potentially useful for optoelectronic applications.     

Gating of Single‐Layer Graphene with Single‐Stranded Deoxyribonucleic Acids

Patterning of biomolecules on graphene layers could provide new avenues to modulate their electrical properties for novel electronic devices. Single‐stranded deoxyribonucleic acids (ssDNAs) are found to act as negative‐potential gating agents that increase the hole density in single‐layer graphene. Current–voltage measurements of the hybrid ssDNA/graphene system indicate a shift in the Dirac point and “intrinsic” conductance after ssDNA is patterned. The effect of ssDNA is to increase the hole density in the graphene layer, which is calculated to be on the order of 1.8 × 10¹² cm^?2. This increased density is consistent with the Raman frequency shifts in the G‐peak and 2D band positions and the corresponding changes in the G‐peak full width at half maximum. Ab initio calculations using density functional theory rule out significant charge transfer or modification of the graphene band structure in the presence of ssDNA fragments.     

Controllable Bipolar Doping of Graphene with 2D Molecular Dopants

The fine control of graphene doping levels over a wide energy range remains a challenging issue for the electronic applications of graphene. Here, the controllable doping of chemical vapor deposited graphene, which provides a wide range of energy levels (shifts up to ± 0.5 eV), is demonstrated through physical contact with chemically versatile graphene oxide (GO) sheets, a 2D dopant that can be solution‐processed. GO sheets are a p‐type dopant due to their abundance of electron‐withdrawing functional groups. To expand the energy window of GO‐doped graphene, the GO surface is chemically modified with electron‐donating ethylene diamine molecules. The amine‐functionalized GO sheets exhibit strong n‐type doping behaviors. In addition, the particular physicochemical characteristics of the GO sheets, namely their sheet sizes, number of layers, and degree of oxidation and amine functionality, are systematically varied to finely tune their energy levels. Finally, the tailor‐made GO sheet dopants are applied into graphene‐based electronic devices, which are found to exhibit improved device performances. These results demonstrate the potential of GO sheet dopants in many graphene‐based electronics applications.     

Controllable,Wide‐Ranging n‐Doping and p‐Doping of Monolayer Group 6 Transition‐Metal Disulfides and Diselenides

Developing processes to controllably dope transition‐metal dichalcogenides (TMDs) is critical for optical and electrical applications. Here, molecular reductants and oxidants are introduced onto monolayer TMDs, specifically MoS₂, WS₂, MoSe₂, and WSe₂. Doping is achieved by exposing the TMD surface to solutions of pentamethylrhodocene dimer as the reductant (n‐dopant) and “Magic Blue,” [N(C₆H₄‐p‐Br)₃]SbCl₆, as the oxidant (p‐dopant). Current–voltage characteristics of field‐effect transistors show that, regardless of their initial transport behavior, all four TMDs can be used in either p‐ or n‐channel devices when appropriately doped. The extent of doping can be controlled by varying the concentration of dopant solutions and treatment time, and, in some cases, both nondegenerate and degenerate regimes are accessible. For all four TMD materials, the photoluminescence intensity; for all four materials the PL intensity is enhanced with p‐doping but reduced with n‐doping. Raman and X‐ray photoelectron spectroscopy (XPS) also provide insight into the underlying physical mechanism by which the molecular dopants react with the monolayer. Estimates of changes of carrier density from electrical, PL, and XPS results are compared. Overall a simple and effective route to tailor the electrical and optical properties of TMDs is demonstrated.     

Few‐Layer Black Phosphorus Carbide Field‐Effect Transistor via Carbon Doping

Black phosphorus carbide (b‐PC) is a new family of layered semiconducting material that has recently been predicted to have the lightest electrons and holes among all known 2D semiconductors, yielding a p‐type mobility (≈10⁵ cm² V^?1 s^?1) at room temperature that is approximately five times larger than the maximum value in black phosphorus. Here, a high‐performance composite few‐layer b‐PC field‐effect transistor fabricated via a novel carbon doping technique which achieved a high hole mobility of 1995 cm² V^?1 s^?1 at room temperature is reported. The absorption spectrum of this material covers an electromagnetic spectrum in the infrared regime not served by black phosphorus and is useful for range finding applications as the earth atmosphere has good transparency in this spectral range. Additionally, a low contact resistance of 289 Ω µm is achieved using a nickel phosphide alloy contact with an edge contacted interface via sputtering and thermal treatment.     

Tuning the Optical and Electrical Properties of Few‐Layer Black Phosphorus via Physisorption of Small Solvent Molecules

Black phosphorus (BP) is recently becoming more and more popular among semiconducting 2D materials for (opto)electronic applications. The controlled physisorption of molecules on the BP surface is a viable approach to modulate its optical and electronic properties. Solvents consisting of small molecules are often used for washing 2D materials or as liquid media for their chemical functionalization with larger molecules, disregarding their ability to change the opto‐electronic properties of BP. Herein, it is shown that the opto‐electronic properties of mechanically exfoliated few‐layer BP are altered when physically interacting with common solvents. Significantly, charge transport analysis in field‐effect transistors reveals that physisorbed solvent molecules induce a modulation of the charge carrier density which can be as high as 10¹² cm^?2 in BP, i.e., comparable to common dopants such as F₄‐TCNQ and MoO₃. By combining experimental evidences with density functional theory calculations, it is confirmed that BP doping by solvent molecules not only depends on charge transfer, but is also influenced by molecular dipole. The results clearly demonstrate how an exquisite tuning of the opto‐electronic properties of few‐layer BP can be achieved through physisorption of small solvent molecules. Such findings are of interest both for fundamental studies and more technological applications in opto‐electronics.