Shuanglong Feng

2D/3D perovskite hybrids as moisture-tolerant and efficient light absorbers for solar cells

The lifetime and power conversion efficiency are the key issues for the commercialization of perovskite solar cells (PSCs). In this paper, the development of 2D/3D perovskite hybrids (CA2PbI4/MAPbIxCl3-x) was firstly demonstrated to be a reliable method to combine their advantages, and provided a new concept for achieving both stable and efficient PSCs through the hybridization of perovskites. 2D/3D perovskite hybrids afforded significantly-improved moisture stability of films and devices without encapsulation in a high humidity of 63 ± 5%, as compared with the 3D perovskite (MAPbIxCl3-x). The 2D/3D perovskite-hybrid film did not undergo any degradation after 40 days, while the 3D perovskite decomposed completely under the same conditions after 8 days. The 2D/3D perovskite-hybrid device maintained 54% of the original efficiency after 220 hours, whereas the 3D perovskite device lost all the efficiency within only 50 hours. Moreover, the 2D/3D perovskite hybrid achieved comparable device performances (PCE: 13.86%) to the 3D perovskite (PCE: 13.12%) after the optimization of device fabrication conditions.

Catalyst-Free, Selective Growth of ZnO Nanowires on SiO2 by Chemical Vapor Deposition for Transfer-Free Fabrication of UV Photodetectors

Catalyst-free, selective growth of ZnO nanowires directly on the commonly used dielectric SiO2 layer is of both scientific significance and application importance, yet it is still a challenge. Here, we report a facile method to grow single-crystal ZnO nanowires on a large scale directly on SiO2/Si substrate through vapor-solid mechanism without using any predeposited metal catalyst or seed layer. We found that a rough SiO2/Si substrate surface created by the reactive ion etching is critical for ZnO growth without using catalyst. ZnO nanowire array exclusively grows in area etched by the reactive ion etching method. The advantages of this method include facile and safe roughness-assisted catalyst-free growth of ZnO nanowires on SiO2/Si substrate and the subsequent transfer-free fabrication of electronic or optoelectronic devices. The ZnO nanowire UV photodetector fabricated by a transfer-free process presented high performance in responsivity, quantum efficiency and response speed, even without any post-treatments. The strategy shown here would greatly reduce the complexity in nanodevice fabrication and give an impetus to the application of ZnO nanowires in nanoelectronics and optoelectronics.

A 3D Chemically Modified Graphene Hydrogel for Fast, Highly Sensitive, and Selective Gas Sensor

Reduced graphene oxide (RGO) has proved to be a promising candidate in high‐performance gas sensing in ambient conditions. However, trace detection of different kinds of gases with simultaneously high sensitivity and selectivity is challenging. Here, a chemiresistor‐type sensor based on 3D sulfonated RGO hydrogel (S‐RGOH) is reported, which can detect a variety of important gases with high sensitivity, boosted selectivity, fast response, and good reversibility. The NaHSO3 functionalized RGOH displays remarkable 118.6 and 58.9 times higher responses to NO2 and NH3, respectively, compared with its unmodified RGOH counterpart. In addition, the S‐RGOH sensor is highly responsive to volatile organic compounds. More importantly, the characteristic patterns on the linearly fitted response–temperature curves are employed to distinguish various gases for the first time. The temperature of the sensor is elevated rapidly by an imbedded microheater with little power consumption. The 3D S‐RGOH is characterized and the sensing mechanisms are proposed. This work gains new insights into boosting the sensitivity of detecting various gases by combining chemical modification and 3D structural engineering of RGO, and improving the selectivity of gas sensing by employing temperature dependent response characteristics of RGO for different gases.

Epitaxial growth of successive CdSe ultrathin films and quantum dot layers on TiO2 nanorod arrays for photo-electrochemical cells

In this work, successive cadmium selenide (CdSe) ultrathin films and quantum dot layers were successfully deposited on TiO2 nanorod arrays by the electrochemical atomic layer epitaxy method (ECALE). The underpotential deposition (UPD) processes of the successive CdSe films and quantum dot layers were recorded in detail. The photo-electrochemical properties of the CdSe coated TiO2 nanorod array electrodes were also investigated, and the maximum current density reached 14.6 mA cm−2 under one sun (AM 1.5G, 100 mW cm−2). Using the ECALE method to grow a buffer layer between quantum dots and their supporting material will be useful for other energy-providing materials.

Bandgap modulation of MoS2 monolayer by thermal annealing and quick cooling

We developed a non-mechanical straining method to simultaneously modulate the bandgap and photoluminescence (PL) quantum efficiency of a synthesized molybdenum disulfide (MoS2) monolayer on SiO2, by vacuum annealing and subsequent quick cooling in ethanol. Influences of the thermal treatments at different temperatures from 100 °C to 600 °C on the PL and Raman spectra of the MoS2 monolayers are reported. A maximum PL peak intensity, twice that of the untreated counterparts under the same measurement conditions, was observed at the treating temperature of 200 °C. At the same time, approximately permanent tensile strains were induced, due to the quick cooling from high temperatures, which led to a red-shift of the direct optical bandgap. Modulation of the bandgap was achieved by changing the treating temperatures; nearly linear PL and Raman frequency shifts of ∼3.82 meV per 100 °C and ∼-0.28 cm-1/100 °C for A exciton photoluminescence and Raman E12g mode frequency were observed, respectively. The proposed thermal modulation promises a wide range of applications in functional 2D nanodevices and semiconductors. To our knowledge, our findings constitute the first demonstration of thermal engineering by combinational manipulation of annealing and quick cooling of the 2D transition-metal dichalcogenides.

Centimeter-Scale Subwavelength Photolithography Using Metal-Coated Elastomeric Photomasks with Modulated Light Intensity at the Oblique Sidewalls

By coating polydimethylsiloxane (PDMS) relief structures with a layer of opaque metal such as gold, the incident light is strictly allowed to pass through the nanoscopic apertures at the sidewalls of PDMS reliefs to expose underlying photoresist at nanoscale regions, thus producing subwavelength nanopatterns covering centimeter-scale areas. It was found that the sidewalls were a little oblique, which was the key to form the nanoscale apertures. Two-sided and one-sided subwavelength apertures can be constructed by employing vertical and oblique metal evaporation directions, respectively. Consequently, two-line and one-line subwavelength nanopatterns with programmable feature shapes, sizes, and periodicities could be produced using the obtained photomasks. The smallest aperture size and line width of 80 nm were achieved. In contrast to the generation of raised positive photoresist nanopatterns in phase shifting photolithography, the recessed positive photoresist nanopatterns produced in this study provide a convenient route to transfer the resist nanopatterns to metal nanopatterns. This nanolithography methodology possesses the distinctive advantages of simplicity, low cost, high throughput, and nanoscale feature size and shape controllability, making it a potent nanofabrication technique to enable functional nanostructures for various potential applications.

Hierarchical double-layered SnO2 film as a photoanode for dye-sensitized solar cells

In this paper, a self-assembling double-layered film consisting of SnO2 nanosheet arrays as an underlayer and hierarchical SnO2 microspheres as an overlayer was fabricated via a facile hydrothermal process. The effect of experimental parameters, such as seed layer, acetylacetone, and NH4F on the morphology of the SnO2 hierarchical double-layered film was investigated. We disclose that these hierarchical structures are the consequence of the simultaneous processes of growing and recrystallization on the FTO seeded surface and in solution. A formation mechanism was proposed to understand the growth process. The reflectance spectra of SnO2 double-layered films were also examined.The novel SnO2-based film shows an enhanced light-to-electricity conversion efficiency compared with a simple composite of SnO2 nanosheet arrays and microspheres due to their self-assembling capability and favorable nanostructures.

Ultrafast growth of large-area monolayer MoS2 film via gold foil assistant CVD for a highly sensitive photodetector

Two-dimensional molybdenum disulfide (MoS2) is a promising material for ultrasensitive photodetector owing to its tunable band gap and high absorption coefficient. However, controlled synthesis of high quality, large area monolayer molybdenum disulfide (MoS2) is still a challenge in practical application. In this work, we report a gold foil assistant chemical vapor deposition (CVD) method of large size (>400 μm) single crystal MoS2 film on silicon dioxide (SiO2) substrate. The influence of Au foil in enlarging the size of single crystal MoS2 were investigated systemically using thermal simulation in Ansys workbench 16.0, including thermal conductivity, temperature difference and thermal relaxation time of the interface of SiO2 substrate and Au foil, which indicated that Au foil could increase the temperature of the SiO2 substrate rapidly and decrease the temperature difference between the oven and substrate. At last, the property of the monolayer MoS2 film was further confirmed by the back-gated field effect transistors (FETs), a high photo-response of 15.6 A/W and a fast photo-response time of 100 ms was obtained. The growth techniques described in this study could be benefit for the development of other atomically thin two-dimensional transition metal dichalcogenides (TMD) materials.

High-efficiency piezoelectric micro harvester for collecting low-frequency mechanical energy

A single-layer zinc oxide (ZnO) nanorod array-based micro energy harvester was designed and integrated with a piezoelectric metacapacitor. The device presents outstanding low-frequency (1-10 Hz) mechanical energy harvesting capabilities. When compared with conventional pristine ZnO nanostructured piezoelectric harvesters or generators, both open-circuit potential and short-circuit current are significantly enhanced (up to 3.1 V and 124 nA cm-2) for a single mechanical knock (∼34 kPa). Higher electromechanical conversion efficiency (1.3 pC/Pa) is also observed. The results indicate that the integration of the piezoelectric metacapacitor is a crucial factor for improving the low-frequency energy harvesting performance. A double piezoelectric-driven mechanism is proposed to explain current higher output power, in which the metacapacitor plays the multiple roles of charge pumping, storing and transferring. An as-fabricated prototype device for lighting an LED demonstrates high power transference capability, with over 95% transference efficiency to the external load.

Ultrafast UV response detectors based on multi-channel ZnO nanowire networks

An UV detector based on multi-channel three dimensional ZnO nanowire networks was fabricated via a catalyst-free CVD method. The ultrafast response time of 20 ms for UV detection was tested in detail. The result revealed that the formation of multi-channels on lateral growth ZnO nanowire arrays can construct a transmission path for UV excited electrons, which is essential for gaining outstanding UV detecting performance. This work not only reports a new way to fabricate in situ nanowire network UV sensors on a chip with large-scale periodic microstructures and a single optical-lithography step via a catalyst free and well controlled synthesis method, but it also confirms a novel mechanism for achieving ultrafast UV detection.