Heterogeneous integration of nanomaterials has enabled advanced electronics and photonics applications. However, similar progress has been challenging for thermal applications, in part due to shorter wavelengths of heat carriers (phonons) compared to electrons and photons. Here, we demonstrate unusually high thermal isolation across ultrathin heterostructures, achieved by layering atomically thin two-dimensional (2D) materials. We realize artificial stacks of monolayer graphene, MoS2, and WSe2 with thermal resistance greater than 100 times thicker SiO2 and effective thermal conductivity lower than air at room temperature. Using Raman thermometry, we simultaneously identify the thermal resistance between any 2D monolayers in the stack. Ultrahigh thermal isolation is achieved through the mismatch in mass density and phonon density of states between the 2D layers. These thermal metamaterials are an example in the emerging field of phononics and could find applications where ultrathin thermal insulation is desired, in thermal energy harvesting, or for routing heat in ultracompact geometries.

Two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDs) are good candidates for high-performance flexible electronics. However, most demonstrations of such flexible field-effect transistors (FETs) to date have been on the micron scale, not benefitting from the short-channel advantages of 2D-TMDs. Here, we demonstrate flexible monolayer MoS2 FETs with the shortest channels reported to date (down to 50 nm) and remarkably high on-current (up to 470 µA µm -1 at 1 V drain-to-source voltage) which is comparable to flexible graphene or crystalline silicon FETs. This is achieved using a new transfer method wherein contacts are initially patterned on the rigid TMD growth substrate with nanoscale lithography, then coated with a polyimide (PI) film which becomes the flexible substrate after release, with the contacts and TMD. We also apply this transfer process to other TMDs, reporting the first flexible FETs with MoSe2 and record on-current for flexible WSe2 FETs. These achievements push 2D semiconductors closer to a technology for low-power and high-performance flexible electronics.For several years, the "Internet-of-Things" (IoT) has been increasingly prevalent in the forecast of future electronics. From monitoring the environment and machines around us to the human body, IoT envisions electronics physically present in every aspect of our daily lives. While some devices may be realized on rigid silicon, there is a need for electronics with new non-planar form factors 1,2 , which are thin and light, and can be conformally attached to objects with unusual shapes, on the human skin, or even implanted into the human body 1 . With these applications in mind, we need to realize electronics on flexible substrates that are robust to mechanical strain, easy to integrate, and capable of low-power consumption and high performance at the nanoscale 2,3 .Recent studies have suggested that 2D materials are good candidates for flexible substrates, because of their lack of dangling bonds, good carrier mobility in atomically thin (sub-1 nm) layers, reduced

Understanding growth, grain boundaries (GBs), and defects of emerging two-dimensional (2D) materials is key to enabling their future applications. For quick, nondestructive metrology, many studies rely on confocal Raman spectroscopy, the spatial resolution of which is constrained by the diffraction limit (∼0.5 μm). Here we use tip-enhanced Raman spectroscopy (TERS) for the first time on synthetic MoSe2 monolayers, combining it with other scanning probe microscopy (SPM) techniques, all with sub-20 nm spatial resolution. We uncover strong nanoscale heterogeneities in the Raman spectra of MoSe2 transferred to gold substrates [one near 240 cm–1 (A1′), and others near 287 cm–1 (E′), 340 cm–1, and 995 cm–1], which are not observable with common confocal techniques and appear to imply the presence of nanoscale domains of MoO3. We also observe strong tip-enhanced photoluminescence (TEPL), with a signal nearly an order of magnitude greater than the far-field PL. Combining TERS with other SPM techniques, we find that GBs can cut into larger domains of MoSe2, and that carrier densities are higher at GBs than away from them.

We report plasmonic enhancement of photocatalysis by depositing 5 nm Au nanoislands onto tungsten diselenide (WSe2) monolayer films. Under 532 nm wavelength illumination, the bare WSe2 film produces a relatively small photocurrent (20 nA). With the addition of Au nanoparticles, we observe enhancements of up to 7× (0.14 μA) in the measured photocurrent. Despite these relatively small photocurrents, it is remarkable that adequate charge separating fields are generated over just 7.3 Å of material. Here, the improvement in the photocatalytic performance is caused by the local electric field enhancement produced in the monolayer WSe2 monolayer by the plasmonic Au nanoislands, as verified by electromagnetic simulations using the finite different time domain (FDTD) method. The near-field optical enhancement increases the electron–hole pair generation rate at the surface of WSe2, thus, increasing the amount of photogenerated charge contributing to photoelectrochemical reactions. Despite reducing the effective surface area of WSe2 in contact with the electrolytic solution by 70%, the plasmonic nanoislands couple the incident light very effectively from the far field to the near field in the plane of the monolayer WSe2, thereby improving the overall photoconversion efficiency from 3.5% to 24.7%.

Two-dimensional (2D) transition-metal dichalcogenides (TMDCs) have been explored for many optoelectronic applications. Most of these applications require them to be on insulating substrates. However, for many fundamental property characterizations, such as mapping surface potential or conductance, insulating substrates are nonideal as they lead to charging and doping effects or impose the inhomogeneity of their charge environment on the atomically thin 2D layers. Here, we report a simple method of residue-free dry transfer of 2D TMDC crystal layers. This method is enabled via noble-metal (gold, silver) thin films and allows comprehensive nanoscale characterization of transferred TMDC crystals with multiple scanning probe microscopy techniques. In particular, intimate contact with underlying metal allows efficient tip-enhanced Raman scattering characterization, providing high spatial resolution (<20 nm) for Raman spectroscopy. Further, scanning Kelvin probe force microscopy allows high-resolution mapping of surface potential on transferred crystals, revealing their spatially varying structural and electronic properties. The layer-dependent contact potential difference is clearly observed and explained by charge transfer from contacts with Au and Ag. The demonstrated sample preparation technique can be generalized to probe many different 2D material surfaces and has broad implications in understanding of the metal contacts and buried interfaces in 2D material-based devices.

Atomically thin semiconductors are of interest for future electronics applications, and much attention has been given to monolayer (1L) sulfides, such as MoS 2 , grown by chemical vapor deposition (CVD). However, reports on the electrical properties of CVD-grown selenides, and MoSe 2 in particular, are scarce. Here, we compare the electrical properties of 1L and bilayer (2L) MoSe 2 grown by CVD and capped by sub-stoichiometric AlO x . The 2L channels exhibit ∼20× lower contact resistance ( R C ) and ∼30× larger current density compared with 1L channels. R C is further reduced by >5× with AlO x capping, which enables improved transistor current density. Overall, 2L AlO x -capped MoSe 2 transistors (with ∼500 nm channel length) achieve improved current density (∼65 μA/μm at V DS = 4 V), a good I on / I off ratio of >10 6 , and an R C of ∼60 kΩ·μm. The weaker performance of 1L devices is due to their sensitivity to processing and ambient. Our results suggest that 2L (or few layers) is preferable to 1L for improved electronic properties in applications that do not require a direct band gap, which is a key finding for future two-dimensional electronics.

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We report a comparison of the photocatalytic performance of WSe 2 -on-MoSe 2 and MoSe 2 -on-WSe 2 heterostructures. While built-in electric fields exist in these heterostructures on the order of 100 kV/cm due to band offsets between these two materials, the photocatalytic performance (i.e., photocurrent) is independent of the stacking order of the two materials. Solving Poisson's equation under these conditions, we find that the built-in electric field produced in the heterostructure is at least 1 order of magnitude smaller than that produced in the electrochemical double layer (i.e., Helmholtz layer). Mott−Schottky measurements indicate that transition metal dichalcogenides (TMDCs) on ITO electrodes have similar capacitance to that of bare ITO, providing further evidence that the interfacial electric fields produced in the solid state heterostructure are negligible compared to the fields generated by the ions in solution. The photocatalytic performance of these heterostructures provided the largest relative enhancement in the heterojunction region under 920 and 785 nm irradiation compared with 532 and 633 nm wavelength excitation. Here, the 920 nm (1.35 eV) photons lie below the band gaps and produce very little photocurrent in the constituent monolayer materials but resonantly excite the interlayer optical transition in the heterostructure, producing a 5-fold enhancement in the measured photocurrent.