Atomically thin two-dimensional semiconductors such as MoS 2 hold great promise in electrical, optical, and mechanical devices and display novel physical phenomena. However, the electron mobility of mono-and few-layer MoS 2 has so far been substantially below theoretically predicted limits, which has hampered efforts to observe its intrinsic quantum transport behaviours. Potential sources of disorder and scattering include both defects such as sulfur vacancies in the MoS 2 itself, and extrinsic sources such as charged impurities and remote optical phonons from oxide dielectrics. To reduce extrinsic scattering, here we developed a van der Waals heterostructure device platform where MoS 2 layers are fully encapsulated within hexagonal boron nitride, and electrically contacted in a multi-terminal geometry using gate-tunable graphene electrodes. Magneto-transport measurements show dramatic improvements in performance, including a record-high Hall mobility reaching 34,000 cm 2 /Vs for 6-layer MoS 2 at low temperature, confirming that low-temperature performance in previous studies was limited by extrinsic interfacial impurities rather than bulk defects in the MoS 2 . We also observed Shubnikov-de Haas oscillations for the first time in high-mobility monolayer and few-layer MoS 2 . Modeling of potential scattering sources and quantum lifetime analysis indicate that a combination of short-ranged and long-ranged interfacial scattering limits low-temperature mobility of MoS 2 . 3Following the many advances in basic science and applications of graphene, other twodimensional (2D) materials, especially transition metal dichalcogenides (TMDCs), have attracted significant interest for their fascinating electrical, optical, and mechanical properties [1][2][3][4][5][6][7][8] . Among the TMDCs, semiconducting MoS 2 has been the mostly widely studied: it shows a thicknessdependent electronic band structure 3,5 , reasonably high carrier mobility 1,2,6-9 , and novel phenomena such as coupled spin-valley physics and the valley Hall effect 10-14 , leading to various applications, such as transistors 1,7,15 , memories 16 , logic circuits 17,18 , light-emitters 19 , and photo-detectors 20 with flexibility and transparency 2,21 . However, as for any 2D material, the electrical and optical properties of MoS 2 are strongly affected by impurities and its dielectric environment 1,2,9,22 , hindering the study of intrinsic physics and limiting the design of 2D-material-based devices. In particular, the theoretical upper bound of the electron mobility of monolayer (1L) MoS 2 is predicted to be from several tens to a few thousands at room temperature (T) and exceed 10 5 cm 2 /Vs at low T depending on the dielectric environment, impurity density and charge carrier density [23][24][25] . In contrast, experimentally measured 1L MoS 2 devices on SiO 2 substrates have exhibited room-T two-terminal field-effect mobility that ranges from 0.1 -55 cm 2 /Vs 1,26,27 . This value increases to 15 -60 cm 2 /Vs with encapsulation by highdielectric materials 1...

We report a determination of the complex in-plane dielectric function of monolayers of four transition metal dichalcogenides: MoS 2 , MoSe 2 , WS 2 and WSe 2 , for photon energies from 1.5-3 eV. The results were obtained from reflection spectra using a Kramers-Kronig constrained variational analysis. From the dielectric functions, we obtain the absolute absorbance of the monolayers. We also provide a comparison of the dielectric function for the monolayers with the corresponding bulk materials.

Emerging two-dimensional (2D) semiconductors such as molybdenum disulfide (MoS2) have been intensively studied because of their novel properties for advanced electronics and optoelectronics. However, 2D materials are by nature sensitive to environmental influences, such as temperature, humidity, adsorbates, and trapped charges in neighboring dielectrics. Therefore, it is crucial to develop device architectures that provide both high performance and long-term stability. Here we report high performance of dual-gated van der Waals (vdW) heterostructure devices in which MoS2 layers are fully encapsulated by hexagonal boron nitride (hBN) and contacts are formed using graphene. The hBN-encapsulation provides excellent protection from environmental factors, resulting in highly stable device performance, even at elevated temperatures. Our measurements also reveal high-quality electrical contacts and reduced hysteresis, leading to high two-terminal carrier mobility (33-151 cm(2) V(-1) s(-1)) and low subthreshold swing (80 mV/dec) at room temperature. Furthermore, adjustment of graphene Fermi level and use of dual gates enable us to separately control contact resistance and threshold voltage. This novel vdW heterostructure device opens up a new way toward fabrication of stable, high-performance devices based on 2D materials.

Atomically thin materials such as graphene and semiconducting transition metal dichalcogenides (TMDCs) have attracted extensive interest in recent years, motivating investigation into multiple properties. In this work, we demonstrate a refined version of the optothermal Raman technique 39,40 to measure the thermal transport properties of two TMDC materials, MoS 2 and MoSe 2 , in single-layer (1L) and bi-layer (2L) forms. This new version incorporates two crucial improvements over previous implementations. First, we utilize more direct measurements of the optical absorption of the suspended samples under study and find values ~40% lower than previously assumed. Second, by comparing the response of fully supported and suspended samples using different laser spot sizes, we are able to independently measure the interfacial thermal conductance to the substrate and the lateral thermal conductivity of the supported and suspended materials. The approach is validated by examining the response of a suspended film illuminated in different radial positions. For 1L MoS 2 and MoSe 2 , the roomtemperature thermal conductivities are (84±17) W/mK and (59±18) W/mK, respectively. For 2L MoS 2 and MoSe 2 , we obtain values of (77±25) W/mK and (42±13) W/mK. Crucially, the interfacial thermal conductance is found to be of order 0.1-1 MW/m 2 K, substantially smaller than previously assumed, a finding that has important implications for design and modeling of electronic devices.

Chronic cranial window (CCW) is an essential tool in enabling longitudinal imaging and manipulation of various brain activities in live animals. However, an active CCW capable of sensing the concealed in vivo environment while simultaneously providing longitudinal optical access to the brain is not currently available. Here we report a disposable ultrasound-sensing CCW (usCCW) featuring an integrated transparent nanophotonic ultrasonic detector fabricated using soft nanoimprint lithography process. We optimize the sensor design and the associated fabrication process to significantly improve detection sensitivity and reliability, which are critical for the intend longitudinal in vivo investigations. Surgically implanting the usCCW on the skull creates a self-contained environment, maintaining optical access while eliminating the need for external ultrasound coupling medium for photoacoustic imaging. Using this usCCW, we demonstrate photoacoustic microscopy of cortical vascular network in live mice over 28 days. This work establishes the foundation for integrating photoacoustic imaging with modern brain research.