The p-n junction diode and field-effect transistor are the two most ubiquitous building blocks of modern electronics and optoelectronics. In recent years, the emergence of reduced dimensionality materials has suggested that these components can be scaled down to atomic thicknesses. Although high-performance fieldeffect devices have been achieved from monolayered materials and their heterostructures, a p-n heterojunction diode derived from ultrathin materials is notably absent and constrains the fabrication of complex electronic and optoelectronic circuits. Here we demonstrate a gate-tunable p-n heterojunction diode using semiconducting single-walled carbon nanotubes (SWCNTs) and single-layer molybdenum disulfide as p-type and n-type semiconductors, respectively. The vertical stacking of these two direct band gap semiconductors forms a heterojunction with electrical characteristics that can be tuned with an applied gate bias to achieve a wide range of charge transport behavior ranging from insulating to rectifying with forward-to-reverse bias current ratios exceeding 10 4 . This heterojunction diode also responds strongly to optical irradiation with an external quantum efficiency of 25% and fast photoresponse <15 μs. Because SWCNTs have a diverse range of electrical properties as a function of chirality and an increasing number of atomically thin 2D nanomaterials are being isolated, the gate-tunable p-n heterojunction concept presented here should be widely generalizable to realize diverse ultrathin, highperformance electronics and optoelectronics.2D transition metal dichalcogenide | single layer MoS 2 | van der Waals heterostructure | rectifier | photodetector

The mechanisms underlying the intrinsic photoresponse of few-layer (FL) molybdenum disulphide (MoS 2 ) field-effect transistors are investigated via scanning photocurrent microscopy.We attribute the locally enhanced photocurrent to band-bending assisted separation of photoexcited carriers at the MoS 2 /Au interface. The wavelength-dependent photocurrents of few layer MoS 2 transistors qualitatively follow the optical absorption spectra of MoS 2 , providing direct evidence of interband photoexcitation. Time and spectrally resolved photocurrent measurements at varying external electric fields and carrier concentrations establish that drift-diffusion currents dominate photothermoelectric currents in devices under bias. TEXT:The layered transition metal dichalcogenides (TMDCs) 1 have attracted great interest recently due to their intriguing electrical and optical properties. 2-8 Field-effect transistors (FETs) fabricated with single layer (SL) and few layer (FL) MoS 2 have shown both unipolar 9-11 and ambipolar 12 charge transport characteristics with high in-plane electron mobility concurrent with high on/off current ratios 9,13 and large current-carrying capacity 10 . While bulk MoS 2 is an n-type semiconductor with an indirect bandgap of ~1.3 eV 14 , single-layer MoS 2 has a direct bandgap of ~1.8 eV 15,16 , which leads to enhanced photoluminescence (PL) compared to bilayer and thicker samples. The combination of a large bandgap in the visible region, strong photoresponse, light emission, and high field-effect mobility makes MoS 2 a promising 2D semiconductor for a variety of electronic 6,17,18 and optoelectronic 7,19,20 applications. Besides the above mentioned, several other interesting properties such as piezoelectricity, 21 tunability of band gaps and phase transitions with electric field, strain and composition 22-25 have also been predicted for ultrathin TMDCs. Solution processed two dimensional materials, decorated with metal nanoparticles or quantum dots (QDs) have also been heavily investigated as catalysts and electrode materials in photochemical reactions. [26][27][28] The high surface area, coupled with a band gap in visible part of the electromagnetic spectrum, makes these ultrathin dichalcogenide based catalysts attractive candidates for solar water splitting. [29][30][31][32] The interface of these nanoparticles with MoS 2 plays a deterministic role in the charge transfer kinetics and efficiency of such photoelectrochemical reactions. 33,34 Thus, the behavior of photoexcited carriers in a controlled FET geometry is relevant to both optoelectronic and energy conversion applications.The photoresponse of MoS 2 -based optoelectronic devices has been attributed to various mechanisms including photoconductivity, 19 photovoltaic effects, 35 and the photothermoelectric

The thickness-dependent band structure of MoS2 implies that discontinuities in energy bands exist at the interface of monolayer (1L) and multilayer (ML) thin films. The characteristics of such heterojunctions are analyzed here using current versus voltage measurements, scanning photocurrent microscopy, and finite element simulations of charge carrier transport. Rectifying I-V curves are consistently observed between contacts on opposite sides of 1L/ML junctions, and a strong bias-dependent photocurrent is observed at the junction. Finite element device simulations with varying carrier concentrations and electron affinities show that a type II band alignment at single layer/multilayer junctions reproduces both the rectifying electrical characteristics and the photocurrent response under bias. However, the zero-bias junction photocurrent and its energy dependence are not explained by conventional photovoltaic and photothermoelectric mechanisms, indicating the contributions of hot carriers.

We report a one-step direct transfer technique for the fabrication of functional nanoelectronic devices using pristine single-walled carbon nanotubes (SWNTs). Suspended SWNTs grown by the chemical vapor deposition (CVD) method are aligned and directly transferred onto prepatterned device electrodes at ambient temperature. Using this technique, we successfully fabricated SWNT electromechanical resonators with gate-tunable resonance frequencies. A fully suspended SWNT p-n diode has also been demonstrated with the diode ideality factor equal to 1. Our method eliminates the organic residues on SWNTs resulting from conventional lithography and solution processing. The results open up opportunities for the fundamental study of electron transport physics in ultraclean SWNTs and for room temperature fabrication of novel functional devices based on pristine SWNTs.

We report the capacitive spring softening effect observed in single-walled carbon nanotube (SWNT) nanoelectromechanical (NEM) resonators. The nanotube resonators adopt dual-gate configuration with both bottom-gate and side-gate capable of tuning the resonance frequency through capacitive coupling. Interestingly, downward resonance frequency shifting is observed with increasing side-gate voltage, which can be attributed to the capacitive softening of spring constant. Furthermore, in-plane vibrational modes exhibit much stronger spring softening effect than out-of-plan modes. Our dual-gate design should enable the differentiation between these two types of vibrational modes, and open up new possibility for nonlinear operation of nanotube resonators.

Spectroscopic, time-resolved scanning photocurrent microscopy is shown to distinguish the intrinsic photoresponse of monodisperse semiconducting (99%) single-walled carbon nanotubes (SWCNTs) from the extrinsic photoresponse of the substrate. A persistent positive photocurrent induced by near-IR excitation is attributed to the generation of free carriers by inter-band excitation in SWCNTs. For shorter excitation wavelengths, absorption by the Si substrate generates two types of photocurrent: a transient positive photoresponse, identified as a displacement current, and a persistent negative photocurrent that arises from photogating of the SWCNT thin film.