Surface-enhanced Raman scattering (SERS) by gold nanoparticle characterizes dermal thickening by collagen in bleomycin-treated skin ex vivo.

PURPOSE: Current skin imaging modalities, including optical, electron, and confocal microscopy, mostly require tissue fixations that could damage proteins and biological molecules. Live tissue or cell imaging such as ultrasonography and optical coherent microscope may not adequately measure the dynamic spectroscopical changes. Raman spectroscopy has been adopted for skin imaging in vivo, mostly for skin cancer imaging. However, whether the epidermal and dermal thickening in skin could be measured and distinguished by conventional Ramen spectroscopy or the surface-enhanced Raman scattering (SERS), a rapid and label-free method for noninvasive measurement remains unknown. METHODS: Human skin sections from patients of atopic dermatitis and keloid, which represent epidermal and dermal thickening, respectively, were measured by conventional Ramen spectroscopy. In mice, skin sections from imiquimod (IMQ)- and bleomycin (BLE)-treated mice, which reflect the epidermal and dermal thickening, respectively, were measured by SERS, that incorporates gold nanoparticles to generate surface plasma and enhance Raman signals. RESULTS: Conventional Ramen spectroscopy failed to consistently show the Raman shift in human samples among the different groups. SERS successfully revealed a prominent peak around 1300 cm-1 in the IMQ-treated skin; and two significant peaks around 1100 and 1300 cm-1 in BLE-treated group. Further quantitative analysis showed 1100 cm-1 peak was significantly accentuated in the BLE-treated skin than that in control skin. SERS identified in vitro a similar 1100 cm-1 peak in solutions of collagen, the major dermal biological molecules. CONCLUSION: SERS distinguishes the epidermal or dermal thickening in mouse skin with rapid and label-free measures. A prominent 1100 cm-1 SERS peak in the BLE-treated skin may result from collagen. SERS might help precision diagnosis in the future.

Unravelling the effect of sulfur vacancies on the electronic structure of the MoS2 crystal.

Molybdenum disulfide (MoS2) is one of the two-dimensional layered semiconductor transition metal dichalcogenides (TMDCs) with great potential in electronics, optoelectronics, and spintronic devices. Sulfur vacancies in MoS2 are the most prevalent defects. However, the effect of sulfur vacancies on the electronic structure of MoS2 is still in dispute. Here we experimentally and theoretically investigated the effect of sulfur vacancies in MoS2. The vacancies were intentionally introduced by thermal annealing of MoS2 crystals in a vacuum environment. Angle-resolved photoemission spectroscopy (ARPES) was used directly to observe the electronic structure of the MoS2 single crystals. The experimental result distinctly revealed the appearance of an occupied defect state just above the valence band maximum (VBM) and an upward shift of the VBM after creating sulfur vacancies. In addition, density functional theory (DFT) calculations also confirmed the existence of the occupied defect state close to the VBM as well as two deep unoccupied states induced by the sulfur vacancies. Our results provide evidence to contradict that sulfur vacancies indicate the origin of n-type behaviour in MoS2. This work provides a rational strategy for tuning the electronic structures of MoS2.

Are graphene-Bi2Te3 van der Waals heterostructure-based saturable absorbers promising for solid-state Q-switched lasers?

This Letter compared the absorption characteristics of a homemade graphene-Bi2Te3 (G-B) van der Waals heterostructure to a Bi2Te3 topological insulator (TI) with a similar preparation method and number of layers. The results indicate that the G-B heterostructure can tremendously enhance the modulation depth and saturable intensity. In addition, a passively Q-switched laser at 1.06 µm with a G-B heterostructure as a saturable absorber (SA) was demonstrated for the first time, to the best of our knowledge. Compared to Bi2Te3 TI, the G-B heterostructure Q-switched laser had better laser performance, indicating that a G-B heterostructure is a promising SA candidate for a 1 µm laser.

Role of carrier-transfer in the optical nonlinearity of graphene/Bi2Te3 heterojunctions.

Two-dimensional (2D) topological insulators (TIs) have attracted a lot of attention owing to their striking optical nonlinearity. However, the ultra-low saturable intensity (SI) of TIs resulting from the bulk conduction band limits their applications, such as in mode-locking solid-state lasers. In this work, through fabricating a graphene/Bi2Te3 heterojunction which combines monolayer graphene and a Bi2Te3 nanoplate, the optical nonlinearities are analyzed. Moreover, the thickness-dependent characteristics are also investigated by varying the thickness of the Bi2Te3 when synthesizing the heterojunctions. Furthermore, with the aid of the estimated junction electron escape time, a model of the photo-excited carrier-transfer mechanism is proposed and used to describe the phenomena of depression of ultra-low saturable absorption (SA) from the Bi2Te3 bulk band. The increased modulation depth of the graphene/Bi2Te3 heterojunction can accordingly be realized in more detail. In addition, a Q-switched solid-state laser operating at 1064 nm with heterojunction saturable absorbers is built up and characterized for validating the proposed model. The laser performance with varied Bi2Te3 thickness, such as pulse duration and repetition rate, agrees quite well with our proposed model. Our work demonstrates the functionality of optical nonlinear engineering by tuning the thickness of the graphene/Bi2Te3 heterojunction and demonstrates its potential for applications.