No abstract

The difficulty in Schottky barrier height (SBH) control arising from Fermi-level pinning (FLP) at electrical contacts is a bottleneck in designing high-performance nanoscale electronics and optoelectronics based on molybdenum disulfide (MoS). For electrical contacts of multilayered MoS, the Fermi level on the metal side is strongly pinned near the conduction-band edge of MoS, which makes most MoS-channel field-effect transistors (MoS FETs) exhibit n-type transfer characteristics regardless of their source/drain (S/D) contact metals. In this work, SBH engineering is conducted to control the SBH of electrical top contacts of multilayered MoS by introducing a metal-interlayer-semiconductor (MIS) structure which induces the Fermi-level unpinning by a reduction of metal-induced gap states (MIGS). An ultrathin titanium dioxide (TiO) interlayer is inserted between the metal contact and the multilayered MoS to alleviate FLP and tune the SBH at the S/D contacts of multilayered MoS FETs. A significant alleviation of FLP is demonstrated as MIS structures with 1 nm thick TiO interlayers are introduced into the S/D contacts. Consequently, the pinning factor ( S) increases from 0.02 for metal-semiconductor (MS) contacts to 0.24 for MIS contacts, and the controllable SBH range is widened from 37 meV (50-87 meV) to 344 meV (107-451 meV). Furthermore, the Fermi-level unpinning effect is reinforced as the interlayer becomes thicker. This work widens the scope for modifying electrical characteristics of contacts by providing a platform to control the SBH through a simple process as well as understanding of the FLP at the electrical top contacts of multilayered MoS.

Tungsten ditelluride (WTe2) is a layered material that exhibits excellent magnetoresistance and thermoelectric behaviors, which are deeply related with its distorted orthorhombic phase that may critically affect the lattice dynamics of this material. Here, we report comprehensive characterization of Raman spectra of WTe2 from bulk to monolayer using experimental and computational methods. We find that mono and bi-layer WTe2 are easily identified by Raman spectroscopy since two or one Raman modes that are observed in higher-layer WTe2 are greatly suppressed below the noise level in the mono- and bi-layer WTe2, respectively. In addition, the frequency of in-plane A1(7) mode of WTe2 remains almost constant as the layer number decreases, while all the other Raman modes consistently blueshift, which is completely different from the vibrational behavior of hexagonal metal dichalcogenides. First-principles calculation validates experimental results and reveals that anomalous lattice vibrations in WTe2 are attributed to the formation of tungsten chains that make WTe2 structurally one-dimensional.

Mono‐ and few‐layer transition metal dichalcogenides (TMDCs) have been widely used as saturable absorbers for ultrashort laser pulse generation, but their preparation is complicated and requires much expertise. The possible use of bulk‐structured TMDCs as saturable absorbers is therefore a very intriguing and technically important issue in laser technology. Here, for the first time, it is demonstrated that defective, bulk‐structured WTe2 microflakes can serve as a base saturable absorption material for fast mode‐lockers that can produce femtosecond pulses from fiber laser cavities. They have a modulation depth of 2.85%, from which stable laser pulses with a duration of 770 fs are readily obtained at a repetition rate of 13.98 MHz and a wavelength of 1556.2 nm, which is comparable to the performance achieved using mono‐ and few‐layer TMDCs. Density functional theory calculations show that the oxidative and defective surfaces of WTe2 microflakes do not degrade their saturable absorption performance in the near‐infrared range, allowing for a broad range of operative bandwidth. This study suggests that saturable absorption is an intrinsic property of TMDCs without relying on their structural dimensionality, providing a new direction for the development of TMDC‐based saturable absorbers.

Most materials and devices with structurally switchable color features responsive to external stimuli can actively and flexibly display various colors. However, realizing covert–overt transformation behavior, especially switching between transparent and colored states, is more challenging. A composite laminate of soft poly(dimethylsiloxane) (PDMS) with a rigid SiO2‐nanoparticle (NP) structure pattern is developed as a multidimensional structural color platform. Owing to the similarity in the optical properties of PDMS and SiO2 NPs, this device is fully transparent in the normal state. However, as their mechanical strengths differ considerably, upon compressive loading, a buckling‐type instability arises on the surface of the laminate, leading to the generation of 1D or 2D wrinkled patterns in the form of gratings. Finally, an application of the device in which quick response codes are displayed or hidden as covert–overt convertible colored patterns for optical encryption/decryption, showing their remarkable potential for anticounterfeiting applications, is demonstrated.

No abstract

We demonstrate the use of an all-fiberized, mode-locked 1.94 μm laser with a saturable absorption device based on a tungsten disulfide (WS2)-deposited side-polished fiber. The WS2 particles were prepared via liquid phase exfoliation (LPE) without centrifugation. A series of measurements including Raman spectroscopy, scanning electron microscopy (SEM), and atomic force microscopy (AFM) revealed that the prepared particles had thick nanostructures of more than 5 layers. The prepared saturable absorption device used the evanescent field interaction mechanism between the oscillating beam and WS2 particles and its modulation depth was measured to be ~10.9% at a wavelength of 1925 nm. Incorporating the WS2-based saturable absorption device into a thulium-holmium co-doped fiber ring cavity, stable mode-locked pulses with a temporal width of ~1.3 ps at a repetition rate of 34.8 MHz were readily obtained at a wavelength of 1941 nm. The results of this experiment confirm that WS2 can be used as an effective broadband saturable absorption material that is suitable to passively generate pulses at 2 μm wavelengths.

Single-walled carbon nanotubes (swCNTs) and nanowires are strong candidate materials for next-generation devices such as high-mobility field-effect transistors (FETs), [1][2][3][4] ultrasensitive sensors, [5][6][7] and so on. [8] One approach for practical device applications can be thin-film devices based on nanotube/ nanowire networks. [9][10][11][12][13][14][15][16][17][18][19][20][21][22] However, such network-based devices have been suffering from various fundamental limitations. For example, transistors based on swCNT networks usually have a poor on-off ratio due to metallic swCNTs in the network channels. [10,16,18] Furthermore, nanotube/nanowire network-based devices in general exhibit low mobility and conductivity with nanoscale channel width due to the poor scaling behavior of percolated network channels. [13,19] Herein, we present a strategy to solve these fundamental problems simply by controlling the connectivity of swCNT/nanowire networks. In this strategy, ''textured'' network channels were prepared via the directed assembly method [20][21][22][23] and they were utilized to fabricate high-performance network-based devices. Using this strategy, we significantly improved the yield of swCNT network-based FETs with a large on-off ratio without removing metallic swCNTs. Polarized Raman spectroscopy was used to systematically investigate the structures of the textured network devices. [24] Significantly, both experimental and simulation results showed that the conductivity and mobility of textured network devices increased with reduced line width, unlike random network-based or conventional silicon-based devices. It indicates that our strategy can be an ideal solution for the fabrication of nanoscale devices based on swCNT/nanowire networks. Figure 1 shows the schematic diagram depicting the procedure to prepare devices with random or textured swCNT network channels. The basic procedure is similar to that reported previously. [22] In brief, an octadecyltrichlorosilane (OTS) self-assembled monolayer (SAM) with non-polar terminal groups was patterned on a SiO 2 surface using photolithography or e-beam lithography. [22][23] When the substrate was placed in the swCNT suspensions (usually 0.1 mg mL À1 in o-dichlorobenzene), swCNTs were selectively adsorbed onto bare SiO 2 regions up to a full monolayer due to the ''self-limiting'' mechanism. [22,25] We used swCNTs grown via the HiPCO method whose average length was %650 nm. Significantly, swCNTs adsorbed on the substrate self-align to stay only inside the bare surface regions without any external forces. In this case, the final structures of assembled swCNT networks were determined by the shape of surface molecular patterns. When the channel width was much larger than the length of individual swCNTs, swCNTs assembled mostly in random orientation forming random network channels. The textured network channels were prepared using multiple parallel line-shape patterns with an individual line width much smaller than the swCNT length. Note that in the previous meth...