In this study, the ambient condition for the as-coated seed layer (SL) annealing at 350 °C is varied from air or nitrogen to vacuum to examine the evolution of structural and optical properties of ZnO nanorods (NRs). The NR crystals of high surface density (~240 rods/μm2) and aspect ratio (~20.3) show greatly enhanced (002) degree of orientation and crystalline quality, when grown on the SLs annealed in vacuum, compared to those annealed in air or nitrogen ambient. This is due to the vacuum-annealed SL crystals of a highly preferred orientation toward (002) and large grain sizes. X-ray photoelectron spectroscopy also reveals that the highest O/Zn atomic ratio of 0.89 is obtained in the case of vacuum-annealed SL crystals, which is due to the effective desorption of hydroxyl groups and other contaminants adsorbed on the surface formed during aqueous solution-based growth process. Near band edge emission (ultra violet range of 360–400 nm) of the vacuum-annealed SLs is also enhanced by 44% and 33% as compared to those annealed in air and nitrogen ambient, respectively, in photoluminescence with significant suppression of visible light emission associated with deep level transition. Due to this improvement of SL optical crystalline quality, the NR crystals grown on the vacuum-annealed SLs produce ~3 times higher ultra violet emission intensity than the other samples. In summary, it is shown that the ZnO NRs preferentially grow along the wurtzite c-axis direction, thereby producing the high crystalline quality of nanostructures when they grow on the vacuum-annealed SLs of high crystalline quality with minimized impurities and excellent preferred orientation. The ZnO nanostructures of high crystalline quality achieved in this study can be utilized for a wide range of potential device applications such as laser diodes, light-emitting diodes, piezoelectric transducers and generators, gas sensors, and ultraviolet detectors.

2D transition metal dichalcogenides are promising in various electronics and optoelectronics applications and have gained popularity owing to their carrier transport and strong light-matter interactions. To fully realize their potential in field-effect transistors (FETs) and photodetectors, high mobility and high responsivity are imperative. Here, we demonstrate the highest mobility of ~166 cm 2 V −1 s −1 at 200 K for single-layer rhenium diselenide (ReSe 2 ) FETs encapsulated between h-BN flakes at V g = 47 V. The high mobility is attributed to low-resistance contacts of scandium/ gold (Sc/Au), with a low Schottky barrier height and reduced charge scattering platform of h-BN. Further, we elucidated the Schottky-barrier-height dependent high photoresponsivity (~3.2 × 10 6 A W −1 ) of few-layer ReSe 2 (FL-ReSe 2 ) at 532 nm-wavelength laser light on an h-BN substrate with Sc/Au contacts. Moreover, broadband light detection of undoped and Co-doped few-layer (FL) ReSe 2 was performed under different laser wavelengths (400-1100 nm). After the deposition of Co nanoparticles, the photocurrent of FL-ReSe 2 increased due to n-doping, as confirmed by the transfer curves of the FL-ReSe 2 -based undoped and co-doped FETs. Further, the work function decreased from 4.856 to 4.791 eV in FL-ReSe 2 , as measured by Kelvin probe force microscopy. No light signal was observed at 1100 nm for the undoped ReSe 2 (1050 nm < λ cut-off < 1100 nm); however, after doping with Co nanoparticles, the cut-off wavelength exceeded to (λ cut-off > 1100 nm), due to the additional trap states generated in the energy band gap of ReSe 2 after Co doping. Further, the transient response of ReSe 2 and Co + ReSe 2 FETs was estimated so that the rise and decay times are decreased from 1.9 s & 2.7 s to 1.1 s & 1.8 s, respectively. ReSe 2 is therefore a promising semiconducting material for electrical and optoelectrical applications.

The construction of high-speed electronic devices that can be integrated using a single two-dimensional (2D) semiconductor with high performance remains challenging due to the absence of locally selective doping methods. In this study, we have demonstrated that the selective opposite polarities (p-type or n-type) from an intrinsic 2H-MoTe2 field-effect transistor (FET) can be configured through carrier type band modulation in molybdenum ditelluride (MoTe2) caused by the charge storage interface in MoTe2/BN vdW heterostructures upon UV illumination with electrostatic gate bias. With this approach, we demonstrate a lateral p-i-n homojunction diode (p-MoTe2/intrinsic-MoTe2/n-MoTe2) using a single two-dimensional semiconductor (2H-MoTe2) where an intrinsic FET (i-type region) is sandwiched between p- and n-type FETs. Electrical performance of such a p-i-n diode demonstrates an ideal rectifying behavior with a rectification ratio (I f/I r) of up to ∼1.4 × 106 at zero gate bias with an ideal value of the ideality factor of nearly ∼1. To support optoelectrical doping, Kelvin probe force microscopy (KPFM) measurements are performed where p- and n-type MoTe2 channels show work function values of ∼5.0 and ∼ 4.55 eV, respectively, with a built-in potential of ∼450 mV. In the photovoltaic mode, the p-i-n diode shows excellent photodetection properties under an illumination of 600 nm, a maximum value of responsivity of 1.10 A/W, and a specific detectivity value of 3.0 × 1012 Jones. The device shows ultrafast photoresponses, where the response speed (τr/τf) is estimated to be 10/20 ns. The proposed research offers an opportunity for creating stable p-i-n homojunction diodes for high-speed electronics with low power consumption using 2D materials.

A growth scheme at a low processing temperature for high crystalline-quality of ZnO nanostructures can be a prime stepping stone for the future of various optoelectronic devices manufactured on transparent plastic substrates. In this study, ZnO nanorods (NRs) grown by the hydrothermal method at 150 °C through doping of transition metals (TMs), such as Co, Ni, or Co-plus-Ni, on polyethylene terephthalate substrates were investigated by various surface analysis methods. The TM dopants in ZnO NRs suppressed the density of various native defect-states as revealed by our photoluminescence and X-ray photoelectron spectroscopy analysis. Further investigation also showed the doping into ZnO NRs brought about a clear improvement in carrier mobility from 0.81 to 3.95 cm2/V-s as well as significant recovery in stoichiometric contents of oxygen. Ultra-violet photodetectors fabricated with Co-plus-Ni codoped NRs grown on an interdigitated electrode structure exhibited a high spectral response of ~137 A/W, on/off current ratio of ~135, and an improvement in transient response speed with rise-up and fall-down times of ~2.2 and ~3.1 s, respectively.

Noble metal dichalcogenides (NMDs) are two-dimensional (2D) layered materials that exhibit outstanding thickness-dependent tunable-bandgaps that can be suitable for various optoelectronic applications.

As a developing technology for flexible electronic device fabrication, ultra-violet (UV) photodetectors (PDs) based on a ZnO nanostructure are an effective approach for large-area integration of sensors on nonconventional substrates, such as plastic or paper. However, photoconductive ZnO nanorods grown on flexible substrates have slow responses or recovery as well as low spectral responsivity R because of the native defects and inferior crystallinity of hydrothermally grown ZnO nanorods at low temperatures. In this study, ZnO nanorod crystallites are doped with Cu or Ni/Cu when grown on polyethylene terephthalate (PET) substrates in an attempt to improve the performance of flexible PDs. The doping with Ni/Cu or Cu not only improves the crystalline quality but also significantly suppresses the density of deep-level emission defects in as-grown ZnO nanorods, as demonstrated by X-ray diffraction and photoluminescence. Furthermore, the X-ray photoelectron spectroscopy analysis shows that doping with the transition metals significantly increases the oxygen bonding with metal ions with enhanced O/Zn stoichiometry in as-grown nanorods. The fabricated flexible PD devices based on an interdigitated electrode structure demonstrates a very high R of ~123 A/W, a high on-off current ratio of ~130, and a significant improvement in transient response speed exhibiting rise and fall time of ~8 and ~3 s, respectively, by using the ZnO nanorods codoped by Ni/Cu.

We examined the influence of O2 plasma treatment for the ZnO seed layer (SL) crystallites on the material characteristics of ZnO nanorods (NRs) synthesized by the hydrothermal method. Diode photocurrent and photo-response transient characteristics of the p-Si/n-ZnO-NR heterojunction-based ultraviolet (UV) photodetectors were also examined according to the plasma treatment for the SLs. The superior optical properties of NRs were measured from the photoluminescence by exhibiting 4.6 times greater near-band edge emission when grown on the O2-plasma-treated SL. The degree of (002) orientation of the NR crystals was improved from 0.67 to 0.95, as revealed by X-ray diffraction analysis, and a higher NR surface density of ~80 rods/μm2 with a smaller mean diameter of 65 nm were also observed by the SL modification using plasma-treatment. It was shown by X-ray photo-electron spectroscopy that this improvement of NR crystalline quality was due to the recovery of stoichiometric oxygen with significant reduction of oxygenated impurities in the SL crystals and the subsequent low-energy growth mode for the NRs. UV PDs fabricated by the proposed SL plasma treatment technique showed significantly enhanced UV-to-dark current ratio from 2.0 to 83.7 at a forward bias of +5 V and faster photo-response characteristics showing the reduction in recovery time from 16 s to 9 s.

No abstract