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Raman spectroscopy is broadly used in the studies of transition metal dichalcogenides to determine the number of layers or other structural parameters. However, unlike the case of graphene, the Raman spectrum varies greatly depending on the excitation energy, and many unusual effects have been reported. The optical absorption spectrum has many features related to exciton states due to the strong coulomb interaction in these materials, and dramatic resonance effects occur when the excitation energy matches one of these exciton states. Several forbidden Raman modes and some unexplained peaks appear near resonance, and Davydov splitting of some Raman modes is observed. Furthermore, the polarization dependence of the some Raman modes also shows excitation energy dependence. In this review, recent progress in resonance Raman studies on transition metal dichalcogenides and some unresolved issues are reviewed. Copyright © 2017 John Wiley & Sons, Ltd.

The atomic or molecular assembly on 2D materials through the relatively weak van der Waals interaction is quite different from the conventional heteroepitaxy and may result in unique growth behaviors. Here, it is shown that straight 1D cyanide chains display universal epitaxy on hexagonal 2D materials. A universal oriented assembly of cyanide crystals (AgCN, AuCN, and Cu0.5Au0.5CN) is observed, where the chains are aligned along the three zigzag lattice directions of various 2D hexagonal crystals (graphene, h‐BN, WS2, MoS2, WSe2, MoSe2, and MoTe2). The potential energy landscape of the hexagonal lattice induces this preferred alignment of 1D chains along the zigzag lattice directions, regardless of the lattice parameter and surface elements as demonstrated by first‐principles calculations and parameterized surface potential calculations. Furthermore, the oriented microwires can serve as crystal orientation markers, and stacking‐angle‐controlled vertical 2D heterostructures are successfully fabricated by using them as markers. The oriented van der Waals epitaxy can be generalized to any hexagonal 2D crystals and will serve as a unique growth process to form crystals with orientations along the zigzag directions by epitaxy., 1D cyanide chains show universal oriented van der Waals epitaxy on various hexagonal 2D crystals. The chain directions are always aligned along the zigzag lattice directions of the 2D substrates, regardless of differing lattice parameters and surface elements. Stacking‐angle‐controlled vertical 2D heterostructures can also be fabricated using cyanide microwire markers for identification of crystal orientations.

Nanocrystalline ZnO films were deposited onto glass substrates in the temperature range of 473–673 K using pulsed spray pyrolysis. The structural, substructural, and optical properties of the films were investigated by means of X-ray diffraction analysis, Raman scattering, and Fourier transform infrared (FTIR) spectroscopy. The effect of the substrate temperature (Ts) on the coherent scattering domain (CSD) sizes L, microstrains ϵ, and microstress σ grades, and the average density of dislocations ρ in the films were estimated through the broadening of X-ray lines using the Cauchy and Gauss approximations and the threefold function convolution method. The ZnO films grown at Ts = 623–673 K possessed the highest values of L, and the lowest of ϵ, σ, and ρ, indicating high-crystalline quality. The Raman spectra showed peaks located at 95–98, 333–336, 415, 439–442, 572, and 578–584 cm−1, which were interpreted as E2low(Zn), (E2high−E2low), E1(TO), E2high(O), A1(LO) and E1(LO) phonon modes of the ZnO wurtzite phase. The FTIR spectra showed relatively weak signals at 856, 1405, and 1560 cm−1, corresponding to the C–H and C–O stretching modes, in addition to the main Zn–O mode at 475 cm−1, indicating a low content of precursor residues.

In this work, we report on the structural and optical properties of ZnO films deposited by pulsed spray pyrolysis at relatively low temperatures, compatiblewith a large variety of substrates and processing technologies. Crystalline ZnO films were deposited onto glass substrates using zinc acetate dihydrate dissolved in distilled water with concentration of 0.2M. The temperature of the substrate was varied in the range Ts = 473–673K with ΔТ = 50 K. Effect of Ts were investigated by scanning electron microscopy, x-ray diffraction and energy dispersive x-ray, and optical spectroscopies. Also, the influence of Ts on the grain size, phase composition, texture quality, coherent scattering domain size, crystal lattice parameters, chemical composition, ransmission coefficient, and the bang gap of the ZnO films were studied. X-ray diffraction analysis revealed that films were polycrystalline with hexagonal phase and showed as preferential orientation (101) at Ts573 K. Scanning electron microscopy (SEM) measurements showed that the substrate temperature has a strong effect on morphology of the films. Energy dispersive analysis revealed that ZnO films consisted of the non-stoichiometric compounds. Optical measurements showed ZnO films to be highly transparent in the visible region, and optical band gap is shifting from 3.18 eV to 3.30 eV. This research was supported by the Ministry of Education and Science of Ukraine (Grant Numbers 0113U000131, 0112U000772, and the individual grant for D.O.).

The CdTe thin and thick films were obtained by the close spaced vacuum sublimation technique on a glass substrate under the following growth conditions: the evaporator temperature was 620 °C; and the substrate temperature was varied in the range from 250 °C to 550 °C. High purity CdTe powder was used as a charge for evaporation. The Raman spectra were measured using TRIAX 320 and TRIAX 550 spectrometers at room temperature. The 488-nm line and 514.5-nm line of an Ar+ laser and a 785-nm diode laser were used as excitation sources. The signal was collected by the liquid nitrogen cooled charge-coupled-device (CCD) detector. A number of intense Raman peaks at 140, 167, 190, 271, 332 and 493 cm-1 were observed and were interpreted as TO (140 cm-1), 1LO (167 cm-1), 2LO (332 cm-1), 3LO (493 cm-1) phonon modes and plasmon-phonon mode (190 cm-1). The presence of several phonon replicas in the Raman spectra confirms high crystal quality of the samples. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

This is the Cu2ZnSnS4 (CZTS) films were fabricated on glass substrates by sputtering using compound material targets. The fabrication method combines the deposition of metallic precursors and the sulfurization annealing process using S vapor. The optimal precursor sequence for CZTS growth was Cu/SnS/ZnS/Mo. To optimize the composition and the thickness of CZTS, the thickness of Cu layer and precursor was varied. The best cell had an area of 0.185 cm2, Cu/(Zn + Sn) = 0.89, Zn/Sn = 1.04, and S/(metal) = 1.13 and showed an open-circuit voltage (VOC) of 0.66 V, a short-circuit current (JSC) of 13.60 mA cm−2, a fill factor of 43.14% and a conversion efficiency of 3.84%.

In article number 1801370, Soon‐Yong Kwon and co‐workers investigate single‐crystalline WTe2 nanobelts from a eutectic metal alloy, which can be a prominent interconnect candidate because of their superior electrical performance and robustness. The successful choice of new interconnect materials from the current 2D library will drive improvements to existing process technologies and will exploit the potential of the assembly of 2D atomic crystals into atomic crystal‐based systems and circuits.

As the elements of integrated circuits are downsized to the nanoscale, the current Cu‐based interconnects are facing limitations due to increased resistivity and decreased current‐carrying capacity because of scaling. Here, the bottom‐up synthesis of single‐crystalline WTe2 nanobelts and low‐ and high‐field electrical characterization of nanoscale interconnect test structures in various ambient conditions are reported. Unlike exfoliated flakes obtained by the top‐down approach, the bottom‐up growth mode of WTe2 nanobelts allows systemic characterization of the electrical properties of WTe2 single crystals as a function of channel dimensions. Using a 1D heat transport model and a power law, it is determined that the breakdown of WTe2 devices under vacuum and with AlOx capping layer follows an ideal pattern for Joule heating, far from edge scattering. High‐field electrical measurements and self‐heating modeling demonstrate that the WTe2 nanobelts have a breakdown current density approaching ≈100 MA cm−2, remarkably higher than those of conventional metals and other transition‐metal chalcogenides, and sustain the highest electrical power per channel length (≈16.4 W cm−1) among the interconnect candidates. The results suggest superior robustness of WTe2 against high‐bias sweep and its possible applicability in future nanoelectronics.

We successfully grew In-rich In0.8Ga0.2N/GaN quantum dots (QDs) by metal-organic chemical vapor deposition. We obtained uniform QDs with density of 1.3 x 1010/cm2 by optimizing growth conditions. Strong photoluminescence (PL) emission from In-rich InGaN/GaN QDs was observed at room temperature and emission wavelength was varied from 404 nm to 454 nm depending on QD size. It is strongly suggested that QDs leads to higher radiative recombination efficiency from temperature-dependent PL measurement. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

By monitoring the intensity of the weak Raman peak from the Si substrate, the transmittance of the multilayer structure could be deduced. Since the samples are identical, except for the thickness of the top aluminum layer, any difference in the intensity of the Si Raman peak is due to the difference in the transmittance of the top aluminum (oxide) layer. It was found that the intensity decreases monotonically with the aluminum layer thickness, which means that there is a substantial underoxidation. Furthermore, the intensity for the 1.6 nm sample is measurably lower than that for the 0.8 nm sample, which indicates that even the optimized 1.6 nm sample may have some degree of under-oxidation. This work demonstrates that Raman spectroscopy could be a useful tool in probing the oxidation state of the aluminum oxide tunnel barriers.

The 5-layer InAs quantum dot (QD) stack was grown on an (001) InP substrate by low pressuremetalorganic chemical vapor deposition. 40 nm InP spacer layers were inserted between the InAs QDs. The integrated intensity of the photoluminescence peak at 300 K was over 12% of that at 10 K. Farinfrared absorption peaks were observed at 819 cm �1 (101.64 meV) and 518 cm �1 (64.08 meV) from this structure at room temperature by attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy. Raman analysis showed that the peak at 819 cm �1 was attributed to a plasmon related peak in the n-type InP substrate. The absorption peak at 518 cm �1 was regarded as a peak related with intersubband transition in the InAs QDs, suggesting that room temperature operating quantum dot devices may be fabricated.

Hydrogen is a common impurity in semiconductors because it is an essential component in the precursors and carrier gases used during the growth or de-vice processing. The hydrogen diffuses in semiconductors and can terminate dangling bonds or passivate both shal-low and deep level defects, allowing neutral complexes with dopant to be formed [1]. Acceptor passivation by hy-drogen has been a critical issue for many years, especially as it pertains to the formation of Mg–H complexes in p-type GaN. The activation of Mg acceptors can be achieved by post-processes [2, 3]. These post-activation processes break the bonding of Mg–H complexes and ex-tract hydrogen from the p-type GaN layer. For GaN layers grown by metal-organic chemical vapor deposition (MOCVD), it is well known that the primary origin of ac-ceptor passivation is hydrogen that originates from decom-position of NH

The influence of a prealloying process on the formation of MoSe2 and thus on the performance of Cu2ZnSnSe4 (CZTSe) solar cells is investigated using sputtering deposition and post-annealing approaches. The dense alloy layer, which is made by a low-temperature prealloying process, acts as a temporary Se diffusion barrier during a subsequent high-temperature selenization process. The formation of thick interfacial MoSe2 can be suppressed effectively by this temporary barrier, cooperating with subsequent quick formation of compact CZTSe layer. The thickness of interfacial MoSe2 layer in CZTSe solar cells can be tailored by adjusting the preannealing process during selenization. As a consequence, the series resistance of CZTSe solar cells is reduced to a low level (≈0.6 Ω cm2), and the performance of CZTSe solar cells is improved significantly. A CZTSe solar cell with efficiency of 8.7% is fabricated.