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Dendritic patterns generated in non-equilibrium growth processes are prevalent in nature while their formation mechanisms are far from fully understood. Here, we report a coverage-dependent fractal degree evolution of monolayer 2H-MoS2 dendrites synthesized on a symmetry-disparate substrate of SrTiO3 (001). Surprisingly, various characterizations have revealed that the monolayer dendrites featured with orthogonal backbones are single crystalline, possessing both peculiar adlayer-substrate interaction and abnormal indirect bandgap on SrTiO3 (001). Further theoretical calculations indicate that a prominent diffusion anisotropy of monomer precursors, combined with the disparate adlayer-substrate symmetry, determine the diffusion-limited aggregation of MoS2 towards dendritic shapes. This work provides brand-new insights in the morphological engineering of two-dimensional atomic crystals, and contributes greatly to an in-depth understanding of the detailed dynamics in non-equilibrium crystal growth.

A facile all-chemical vapor deposition approach is designed, which allows both sequentially grown Gr and monolayer MoS2 in the same growth process, thus allowing the direct construction of MoS2 /Gr vertical heterostructures on Au foils. A weak n-doping effect and an intrinsic bandgap of MoS2 are obtained from MoS2 /Gr/Au via scanning tunneling microscopy and spectroscopy characterization. The exciton binding energy is accurately deduced by combining photoluminescence measurements.

2D metallic transition metal dichalcogenides (MTMDCs) are benchmark systems for uncovering the dimensionality effect on fascinating quantum physics, such as charge-density-wave (CDW) order, unconventional superconductivity, and magnetism, etc. However, the scalable and thickness-tunable syntheses of such envisioned MTMDCs are still challenging. Meanwhile, the origin of CDW order at the 2D limit is controversial. Herein, the direct synthesis of wafer-scale uniform monolayer 2H-TaSe2 films and thickness-tunable flakes on Au foils by chemical vapor deposition is accomplished. Based on the thickness-tunable 2H-TaSe2, the robust periodic lattice distortions that relate to CDW orders by low-temperature transmission electron microscopy are directly visualized. Particularly, a phase diagram of the transition temperature from normal metallic to CDW phases with thickness by variable-temperature Raman characterizations is established. Intriguingly, dramatically enhanced transition temperature from bulk value ≈90 to ≈125 K is observed from monolayer 2H-TaSe2, which can be explained by the enhanced electron-phonon coupling mechanism. More importantly, an ultrahigh specific capacitance is also obtained for the as-grown TaSe2 on carbon cloth as supercapacitor electrodes. The results hereby open up novel avenues toward the large-scale preparation of high-quality MTMDCs, and shed light on their applications in exploring some fundamental issues.

Vapor-liquid-solid processing of boron nanowires (BNWs) can be carried out either using a bottom-up or top-down growth mode, which results in different contact modes between the nanowire and the substrate. The contact mode may strongly affect the electrical transport and field-emission performance of the individual boron nanowires grown on a Si substrate. The electrical transport and field-emission characteristics of individual boron nanowires of different contact modes are investigated in situ using a scanning electron microscope. The contact barriers are very distinct for the different contact modes. Moreover, the transition from a "contact-limited" to a "bulk-limited" field-emission (FE) process is demonstrated in nanoemitters for the first time, and the proposed improved metal-insulator-vacuum (MIV) model may better illustrate the nonlinear behavior of the Fowler-Nordheim (FN) plots in these nanoscale systems. Individual BNWs with different contact modes have a discrepancy in their emission stability and vacuum breakdown characteristics though they have similar aspect ratios, which suggests that their electrical transport and field-emission performance are closely related to their contact mode. Boron nanowires grown in the base-up mode have better field-emission performances and are more beneficial than those grown in the top-down mode for various device applications.

Boron is a narrow-bandgap (1.56 eV) semiconductor with high melting-point, low-density, large Young's modulus and very high refractive index (3.03) close to silicon. Therefore, boron nanostructures is expected to possess strong visible-light scattering properties. However, photonic and optoelectronic properties of the boron nanostructures are seldom studied until now. In this paper, we have successfully prepared single-crystalline boron nanowire (BNW) arrays with high-density on Si substrate. All the BNWs are found to possess strong light-scattering behaviors in the visible regime. Most of all, the scattered light is found to polarize along the longitudinal direction of the nanowire. They also have excellent second-harmonic generation (SHG) properties under ultrafast laser irradiation. Further optoelectronic measurements show that an individual BNW device exhibits notable photocurrent responses in the visible-light range at ambient conditions, which can be attributed to the strong coupling effect between individual BNW and the visible light. The maximum photoresponsivity of an individual BNW can reach up to 12.12 A W-1 at a voltage of 10 V, and the response time is only 18 ms. Therefore, it unveils that the BNWs have a promising future in visible-light communications and detections.

2D van der Waals (vdW) layered polar crystals sustaining phonon polaritons (PhPs) have opened up new avenues for fundamental research and optoelectronic applications in the mid-infrared to terahertz ranges. To date, 2D vdW crystals with PhPs are only experimentally demonstrated in hexagonal boron nitride (hBN) slabs. For optoelectronic and active photonic applications, semiconductors with tunable charges, finite conductivity, and moderate bandgaps are preferred. Here, PhPs are demonstrated with low loss and ultrahigh electromagnetic field confinements in semiconducting vdW α-MoO3 . The α-MoO3 supports strong hyperbolic PhPs in the mid-infrared range, with a damping rate as low as 0.08. The electromagnetic confinements can reach ≈λ0 /120, which can be tailored by altering the thicknesses of the α-MoO3 2D flakes. Furthermore, spatial control over the PhPs is achieved with a metal-ion-intercalation strategy. The results demonstrate α-MoO3 as a new platform for studying hyperbolic PhPs with tunability, which enable switchable mid-infrared nanophotonic devices.

Single crystalline boron nanocones are obtained by a simple chemical vapor deposition method. Electric conductivity values of boron nanocones are (1.0-7.3) x 10(-5) (Omega cm)(-1). Results of field emission show the low turn-on and threshold electric fields of about 3.5 V mu m(-1) and 5.3 V mu m(-1), respectively. Boron nanocones with good electrical transport and field emission properties are promising candidates for application in flat panel displays and nanoelectronics building blocks.

Owing to their unique semiconducting properties, ZnO nanowires (NWs) have received intensive research interests in optics, electronics, piezoelectrics, and mechanics. The vertically aligned morphology has already shown great advantages in manipulation and device assembly within the nanometer regime. Among them, field emission (FE) is one of the most important applications since the emitting efficiency can be highly improved by the alignment. Comprehensive theoretical and experimental research on FE has been mainly conducted on carbon nanotubes (CNTs) owing to their good conductivity, chemical stability, and easy as well as cost-effective fabrication. It has been discovered that the emitter density is very critical to the emission efficiency. An array of densely packed CNTs greatly reduces the field-enhancement factor at the CNT tip to a level not much different from a flat metal plate. Too loosely distributed CNTs cannot meet the desired requirement of high current density and high-emitting points. Oxide semiconducting NWs, which are more stable at high temperatures in an oxygen environment and have a more controllable electronic property, have been considered more and more as alternative FE sources instead of CNTs. With a large exciton binding energy and high melting temperature, ZnO NWs have recently been studied as an effective FE source. However, very little research has been performed on optimizing their FE property by controlling the density of the aligned ZnO NWs. Recently, we developed a low-cost self-assembly process to synthesize aligned ZnO NWs with controlled sizes, lengths, and densities. Moreover, with the simultaneous formation of a continuous conductive layer at the roots, these NW arrays exhibit great merit for device applications, such as FE. In a FE measurement, the distance between the sample and counter electrode is an important parameter to determine the field-enhancement factor. However, this distance is usually difficult to precisely determine inside a vacuum-testing chamber. In order to perform an accurate investigation, we developed an in situ measuring system inside a scanning electron microscope chamber, in which the emitting distance could be directly observed. The working vacuum of the chamber at ca. 5 × 10 Torr (1 Torr = 133.322 Pa) provides a reasonable vacuum condition for emission testing. This system provided a novel and reliable way to measure the FE property with knowledge of the exact emitting distance, nanowire density, and the region being tested. In this paper, we report such in situ measurements on a series of aligned ZnO NWs with controlled density and size. It was found that the NWs with a density between 60 and 80 lm and a length of ca. 1 lm gave the highest emitting current, which was ca. 20 lA at a field of 10 Vlm, detected by using a 0.03 mm W counter electrode. The setup of the in situ measuring system is schematically shown in Figure 1a and detailed information is presented in C O M M U N IC A IO N

W18O49 nanowires were synthesized by a high-temperature physical evaporation technique. The structure, morphology, and composition of the nanowires were characterized by SEM, EMPA, XRD, XPS, and HRTEM techniques. The intrinsic Raman spectrum of W18O49 nanowires was obtained, and the effect of laser power on the change of their structure was also studied. W18O49 nanowires were first oxidized to tungsten trioxide nanowires under irradiation of a certain laser power, and then the tungsten trioxide nanowires underwent a phase transition from monoclinic to orthorhombic with increasing laser power; this phase transition was reversible on turning down the laser power. Copyright © 2006 John Wiley & Sons, Ltd.

Fabrication of nanoscale biosensors based on nanowires (NWs), nanotubes (NTs), and other nanomaterials has recently attracted enormous attention. In comparison to nanoparticles, 1D NWs and NTs have higher sensitivity because of depletion or accumulation of charge carriers at the surface that is caused by binding of charged biological macromolecules at the surface, and affects the entire cross-sectional conduction pathway. Among all 1D nanomaterials, Si NWs and carbon NTs are the most studied materials as biosensors. Functionalized Si NWs and carbon NTs have been demonstrated for detecting proteins, DNA and DNA sequence variations, and cancer markers. However, the biocompatibility and biodegradability of these nanostructures remain to be studied. For example, carbon NTs injected into human blood vessels might accumulate and occlude capillaries in the human brain, which could cause serious damage or be fatal. Being a key functional material with versatile properties, such as dual semiconducting and piezoelectric properties, ZnO has important applications in optoelectronic devices, sensors, lasers, transducers, and photovoltaic devices. In addition, the morphology and the dopant concentration of ZnO nanostructures can be well controlled by tuning the growth conditions, which further broadens their applications. ZnO nanoparticles are believed to be nontoxic, biosafe, and possibly biocompatible, and have been used in many applications in our daily life, such as drug carriers and cosmetics. However, no literature is available on the biodegradability and biocompatibility of ZnO nanowires or nanobelts, which is crucial for the application of ZnO nanostructure for biosensing. In this paper, we present the first study on biodegradability and biocompatibility of ZnO wires. We have conducted a systematic study on the etching and dissolving behavior of ZnO NWs in various solutions with moderate pH values, including deionized water, ammonia, NaOH solution, and horse blood serum. The result shows that ZnO can be dissolved by deionized water (pH≈ 4.5–5.0), ammonia (pH≈ 7.0–7.1, 8.7–9.0) and NaOH solution (pH≈ 7.0–7.1, 8.7–9.0). The study of the interaction of ZnO wires with horse blood serum shows that the ZnO wires can survive in the fluid for a few hours before they eventually degrade into mineral ions. The results of this study are of great significance. First, biosensors made of ZnO nonmaterial have a certain time to perform a device function. Secondly, once completing the corresponding service, the ZnO wires can eventually dissolve into ions that can be completely absorbed by the body and become part of the nutrition. The biodegradability and biocompatibility of ZnO NWs would allow their use for in vivo biosensing and biodetection. Synthesized by a vapor–solid growth process, the ZnO wires used in our study grew along the [0001] direction with a hexagonal cross section and were of high crystalline quality. We studied the dissolving behavior of ZnO wires in deionized water (pH≈ 4.5–5.0), ammonia (pH≈ 7.0–7.1, 8.7–9.0), NaOH solution (pH≈ 7.0–7.1, 8.7–9.0), horse blood serum solution (pH≈ 7.9–8.2), and pure horse blood serum (pH≈ 8.5). The two kinds of ammonia used in our study were prepared by diluting concentrated ammonia with deionized water. The two kinds of NaOH solution were prepared by dissolving solid NaOH in deionized water, and the horse blood serum solution was prepared by diluting pure horse blood serum with NaOH solution (pH≈ 7.0–7.1) with a volume ratio of 1:10. We adopted two processes to investigate the dissolving behavior of a single ZnO wire in different liquids. To study the dissolving process of ZnO wires in deionized water, ammonia, and NaOH solution, we used Process 1 illustrated in Figure 1a. Individual ZnO NWs were firstly manipulated with a pin and placed on a silicon substrate. After that, a droplet of C O M M U N IC A TI O N S

Details are given of an experimental study of the field emission characteristics of well-aligned carbon nanotube films. The films were prepared in a catalyst-assisted process using bias-enhanced microwave plasma chemical vapour deposition. They consist of bundles of carbon nanotubes, the density of which may be controlled by changing the thickness of the pre-deposited catalyst film. The transparent anode technique was used to measure the field emission properties of the carbon nanotube films and it was found that films having a low density of bundles have better field emission properties. The findings are explained by the electrostatic screen effect. Copyright © 2004 John Wiley & Sons, Ltd.

The development of field electron emission materials requires purposeful analytical modeling of structural features. A vertical wedged ordered nanostructure shows better field emission current density because of its in-plane continuity at the top emission edge: Individually, its calculated average current density can be 8.3–26.7 times larger than a tapered nanostructure when its bottom area s increases from 0.5 to 2.5 µm2, can be 2.2–12.7 times as its radius of top curvature r decreases from 65 to 5 nm, and can be 7.3–17.3 times as its length L increases from 0.5 to 5 µm. For an ordered nanostructure array, its average current density can be about 160 times larger than the a reference tapered array when the individual elements are closely aligned. Over three orders of magnitude improvment can be observed when optimal emitter spacing conditions are used. Structural analytical models and numerical simulation prove to be in good correspondence with previously reported experimental data, and may serve as a guide for the design and preparation of field electron emission materials with a clear purpose in the future.

WO2 nanowires have great potential as cold cathode materials because they exhibit excellent field-emission properties in terms of their low work function and high conductivity. Considering the requirements of the practical applications in large-area cold-cathode devices, such as X-ray source and panel display, these nanostructures are desired to be fabricated on glass substrate and integrated into devices. It is best known that the studies on WO2 nanowire cold cathode arrays grown on glass-substrate are absent so far, which remains a challenge for the researchers. In the current study, procedures for fabrication of WO2 nanowire cold cathode arrays, which consist of 22000 separated patterns on 1.2 in. glass wafer are developed. It is found that the turn-on and threshold field of the nanowire arrays are, respectively, 6.6 and 9.3 V µm−1. In addition, the performance of the cold cathode arrays is strongly affected by the distribution of the nanowire density as well as the thickness of Al electrode. By modifying the fabrication techniques, over 82% patterns can contribute to the emission, whereby a maximum current density can reach 1.89 mA cm−2. Such a value can fulfill the basic requirement of the large-area cold cathode devices. The research can pave the way for fabrication of high-performance cold cathode devices using glass-substrates.