Enhancing the electrical and optoelectronic performance of nanobelt devices by molecular surface functionalization

Ultrahigh anisotropic carrier mobility in ZnSb monolayers functionalized with halogen atoms

The experimental fabrication of novel two-dimensional ZnSb inspires us to explore the tunability of its fundamental physical properties. In this work, we present the density functional theory simulations on the mechanical, electronic and transport properties of the two-dimensional ZnSb monolayers functionalized with halogen atoms. It is found that the halogen atoms prefer to form ionic bonds with Sb atoms and these ZnSbX (X = Cl, Br and I) monolayers are very flexible with Young's moduli ranging from 24.02 N m⁻¹ to 30.16 N m⁻¹ along the armchair and zigzag directions. The pristine ZnSb monolayer sheet exhibits metallic phase while the functionalization can lead to a metal-to-semiconductor transition with band gaps as large as 0.55 eV. The transport study reveals a large tunability with the hole mobility reaching 43.44 × 10³ cm² V⁻¹ s⁻¹ along the armchair direction and the electron mobility as high as 36.99 × 10³ cm² V⁻¹ s⁻¹ along the zigzag direction. In contrast, the electron mobility along the armchair direction and the hole mobility along the zigzag direction are of relatively small magnitude. The ultrahigh carrier mobility together with the directional anisotropy can boost the separation of photo-excited electron–hole pairs. The finite band gaps and exceptional transport property of ZnSbX monolayers render them new materials with promising applications in flexible optoelectronic and nanoelectronic devices.

Ultrahigh carrier mobility and transport anisotropy in ZnSb monolayers functionalized with halogen atoms.

Combining triboelectric nanogenerator with piezoelectric effect for optimizing Schottky barrier height modulation

Schottky-contacted sensors have been demonstrated to show high sensitivity and fast response time in various sensing systems. In order to improve their sensing performance, the Schottky barriers height (SBH) at the interface of semiconductor and metal electrode should be adjusted to appropriate range to avoid low output or low sensitivity, which was induced by excessively high or low SBH, respectively. In this work, a simple and effective SBH tuning method by triboelectric generator (TENG) is proposed, the SBH can be effectively lowered by voltage pulses generated by TENG and gradually recover over time after withdrawing the TENG. Through combining the TENG treatment with piezotronic effect, a synergistic effect on lowering SBH was achieved. The change of SBH is increased by 3.8 to 12.8 times, compared with dependent TENG treatment and piezotronic effect, respectively. Furthermore, the recovery time of the TENG-lowered SBH can be greatly shortened from 1.5 h to 40 s by piezotronic effect. This work demonstrated a flexible and feasible SBH tuning method, which can be used to effectively improve the sensitivity of Schottky-contact sensor and sensing system. Our study also shows great potential in broadening the application scenarios of Schottky-contacted electronic devices.

Ultrasensitive Fiber-Based ZnO Nanowire Network Ultraviolet Photodetector Enabled by the Synergism between Interface and Surface Gating Effects

A flexible UV photodetector with a high on/off ratio is extremely important for environmental sensing, optical communication, and flexible optoelectronic devices. In this work, a flexible fiber-based UV photodetector with an ultrahigh on/off ratio is developed by utilizing the synergism between interface and surface gating effects on a ZnO nanowire network structure. The synergism between two gating effects is realized by the interplay between surface band bending and the Fermi level through the nanowire network structure, which is proved through the control experiments between the ZnO micro/nanowire photodetector and micro/nanowire junction photodetector, and the corresponding Kelvin probe force microscopy (KPFM) measurements. The on/off ratio of the fiber-based ZnO nanowire network UV photodetector reaches 1.98 × 10⁸ when illuminated by 1.0 mW cm^-2 UV light, which is 20 times larger than the largest reported result under the same UV illumination. This new UV sensor also has a high resolution to UV light intensity change in the nW cm^-2 range. Furthermore, when the fiber-based photodetector is curved, it still shows excellent performance as above. This work gives a new effective route for the development of a high-performance UV photodetector or other optoelectronic detection devices.

Enhanced photoresponse of epitaxially grown ZnO by MoO3 surface functionalization

Enhanced photoresponse of epitaxially grown ZnO has been observed with MoO₃ surface functionalization, which is attributed to the larger upward band bending at the interface.

Detection and Discrimination of Volatile Organic Compounds using a Single Film Bulk Acoustic Wave Resonator with Temperature Modulation as a Multiparameter Virtual Sensor Array

This paper describes the detection and discrimination of volatile organic compounds (VOCs) using an e-nose system based on a multiparameter virtual sensor array (VSA), which consists of a single-chip temperature-compensated film bulk acoustic wave resonator (TC-FBAR) coated with 20-bilayer self-assembled poly(sodium 4-styrenesulfonate)/poly(diallyldimethylammonium chloride) thin films. The high-frequency and microscale FBAR multiparameter VSA was realized by temperature modulation, which can greatly reduce the cost and complexity compared to those of a traditional e-nose system and can allow it to operate at different temperatures. The discrimination effect depends on the synergy of temperature modulation and the sensing material. For proof-of-concept validation purposes, the TC-FBAR was exposed to six different VOC vapors at six different gas partial pressures by real-time VOC static detection and dynamic detection. The resulting frequency shifts and impedance responses were measured at different temperatures and evaluated using principal component analysis and linear discriminant analysis, which revealed that all analytes can be distinguished and classified with more than 97% accuracy. To the best of our knowledge, this report is the first on an FBAR multiparameter VSA based on temperature modulation, and the proposed novel VSA shows great potential as a compact and promising e-nose system integrated in commercial electronic products.

Bioelectronics communication: encoding yeast regulatory responses using nanostructured gallium nitride thin films

Baker's yeast, S. cerevisiae, is a model organism that is used in synthetic biology. The work demonstrates how GaN nanostructured thin films can encode physiological responses in S. cerevisiae yeast. The Ga-polar, n-type, GaN thin films are characterized via Photocurrent Measurements, Atomic Force Microscopy and Kelvin Probe Force Microscopy. UV light is used to induce persistent photoconductivity that results in charge accumulation on the surface. The morphological, chemical and electronic properties of the nanostructured films are utilized to activate the cell wall integrity pathway and alter the amount of chitin produced by the yeast. The encoded cell responses are induced by the semiconductor interfacial properties associated with nanoscale topography and the accumulation of charge on the surface that promotes the build-up of oxygen species and in turn cause a hyperoxia related change in the yeast. The thin films can also alter the membrane voltage of yeast. The observed modulation of the membrane voltage of the yeast exposed to different GaN samples supports the notion that the semiconductor material can cause cell polarization. The results thus define a strategy for bioelectronics communication where the roughness, surface chemistry and charge of the wide band gap semiconductor's thin film surface initiate the encoding of the yeast response.

Molecular aggregation of naphthalimide organic semiconductors assisted by amphiphilic and lipophilic interactions: a joint theoretical and experimental study

Amphiphilic and lipophilic donor-acceptor naphthalimide-oligothiophene assemblies exhibiting almost identical intramolecular properties, but differing in their intermolecular interactions, have been synthesized. Here we analyze the effect of replacing the normally used lipophilic alkyl chains with hydrophilic ones in directing molecular aggregation from an antiparallel to a parallel stacking. This different molecular packing of the amphiphilic, NIP-3TAmphi, and lipophilic, NIP-3TLipo, systems is assessed by electronic spectroscopies, scanning electronic microscopy and DFT quantum-chemical calculations. Theoretical calculations indicate that the presence of amphiphilic interactions promotes a face-to-face parallel arrangement of neighbor molecules, which induces improved electronic coupling and therefore enhances the charge transport ability and photoconducting properties of this type of materials. Time of flight and photoconducting measurements are used to determine the impact of the amphiphilic and lipophilic interactions on their possible performance in optoelectronic devices.

Variably doped nanostructured gallium nitride surfaces can serve as biointerfaces for neurotypic PC12 cells and alter their behavior

Neurotypic PC12 cells behavior was studied on nanostructured GaN and rationalized with respect to surface charge, doping level, and chemical functionalization. The semiconductor analysis included atomic force microscopy, Kelvin probe force microscopy, and X-ray photoelectron spectroscopy. The semiconductor surfaces were then evaluated as biointerfaces, and the in vitro cell behavior was quantified based on cell viability, reactive oxygen species production, as well as time dependent intracellular Ca concentration, [Ca²⁺]_i, a known cell-signaling molecule. In this work, we show that persistent photoconductivity (PPC) can be used to alter the surface properties prior to chemical functionalization, the concentration of dopants can have some effect on cellular behavior, and that chemical functionalization changes the surface potential before and after exposure to UV light. Finally, we describe some competing mechanisms of PPC-induced [Ca²⁺]_i changes, and how researchers looking to control cell behavior non-invasively can consider PPC as a useful control knob.

Neurotypic PC12 cells behavior was studied on nanostructured GaN and rationalized with respect to surface charge, doping level, and chemical functionalization.

All-inkjet-printed flexible ZnO micro photodetector for a wearable UV monitoring device

Fabrication of small-sized patterns of inorganic semiconductor onto flexible substrates is a major concern when manufacturing wearable devices for measuring either biometric or environmental parameters. In this study, micro-sized flexible ZnO UV photodetectors have been thoroughly prepared by a facile inkjet printing technology and followed with heat treatments. A simple ink recipe of zinc acetate precursor solution was investigated. It is found that the substrate temperature during zinc precursor ink depositing has significant effects on ZnO pattern shape, film morphology, and crystallization. The device fabricated from the additive manufacturing approach has good bendability, Ohmic contact, short response time as low as 0.3 s, and high on/off ratio of 3525. We observed the sensor's dependence of response/decay time by the illuminating UV light intensity. The whole process is based on additive manufacturing which has many benefits such as rapid prototyping, saving material, being environmentally friendly, and being capable of creating high-resolution patterns. In addition, this method can be applied to flexible substrates, which makes the device more applicable for applications requiring flexibility such as wearable devices. The proposed all-inkjet-printing approach for a micro-sized ZnO UV photodetector would significantly simplify the fabrication process of micro-sized inorganic semiconductor-based devices. A potential application is real-time monitoring of UV light exposure to warn users about unsafe direct sunlight to implement suitable avoidance solutions.

Nanophotoactivity of Porphyrin Functionalized Polycrystalline ZnO Films

Kelvin probe force microscopy in darkness and under illumination is reported to provide nanoscale-resolved surface photovoltage maps of hybrid materials. In particular, nanoscale charge injection and charge recombination mechanisms occurring in ZnO polycrystalline surfaces functionalized with Protoporphyrin IX (H2PPIX) are analyzed. Local surface potential and surface photovoltage maps not only reveal that upon molecular adsorption the bare ZnO work function increases, but also they allow study of its local dependence. Nanometer-sized regions not correlated with apparent topographic features were identified, presenting values significantly different from the average work function. Depending on the region, the response to the light excitation is different, distinguishing two relaxation processes, one faster than the other. This behavior can be explained in terms of electrons trapped closed to the molecule-semiconductor interface or electrons pushed into the ZnO bulk, respectively. Moreover, the origin of these differences is correlated with the H2PPIX-ZnO bonding and molecules configuration and aggregation. The chenodeoxycholic acid (CDCA) coadsorption leads to a more homogeneous surface potential distribution, confirming the antiaggregate effect of this additive, while the surface photovoltage is mostly dominated by the slow relaxation component. This work reveals the complexity of real device architectures with ill-defined surfaces even in a relatively simple system with only one type of dye molecule and hightlights the importance of nanoscale characterization with appropriate tools.

Negative thermal quenching of photoluminescence in annealed ZnO–Al2O3 core–shell nanorods

Negative thermal quenching behavior of photoluminescence is observed in the annealed ZnO–Al₂O₃ core–shell nanorods, which is originated from the Al donor in ZnO induced through an annealing process.

Shottky Contact Device Based on a Single WO3 Nanowire for Ultraviolet Photodetector

To distinguish the ultraviolet (UV) photoresponse of Ohmic and Schottky contact devices, we have fabricated symmetrical and nonsymmetrical devices by standard lithography based on a single WO3 nanowire. For the Ohmic contact device, the photocurrent can change from 100 nA to 300 nA. Even 200 s under UV illumination, nonsaturated photocurrent can be observed, and the fall time is more than 1000 s. But for the Schottky contact device, the rise and fall time are faster than that of Ohmic device. The barrier height of Schottky device can be easily controlled through the oxygen adsorption and desorption on the junction region, which can be served as a ‘‘gate’’ that effectively tunes the conductance of the device. Therefore, the Schottky barrier plays a very important role in the rapid-response of UV photodetector.

Application of three-dimensionally area-selective atomic layer deposition for selectively coating the vertical surfaces of standing nanopillars

We describe a strategy for selectively coating the vertical surfaces of standing nanopillars using area-selective atomic layer deposition (ALD). Hydrophobic self-assembled monolayers (SAMs) are utilised to selectively inhibit the coating of oxides on the modified horizontal regions to ensure that only the vertical surfaces of vertical standing nanorods are coated using ALD processes. This method makes it possible to fabricate vertical nanodevices using a simple process of depositing oxide layer on a vertical surface, and can also be applied to the area-selective surface passivation of other standing structures.

Nanosecond laser switching of surface wettability and epitaxial integration of c-axis ZnO thin films with Si(111) substrates

We have achieved integration of polar ZnO[0001] epitaxial thin films with Si(111) substrates where cubic yttria-stabilized zirconia (c-YSZ) was used as a template on a Si(111) substrate. Using XRD (θ-2θ and φ scans) and HRTEM techniques, the epitaxial relationship between the ZnO and the c-YSZ layers was shown to be [0001]ZnO || [111]YSZ and [21¯1¯0]ZnO || [1¯01](c-YSZ), where the [21¯1¯0] direction lies in the (0001) plane, and the [1¯01] direction lies in the (111) plane. Similar studies on the c-YSZ/Si interface revealed epitaxy as (111)YSZ || (111)Si and in-plane (110)YSZ || (110)Si. HRTEM micrographs revealed atomically sharp and crystallographically continuous interfaces. The ZnO epilayers were subsequently laser annealed by a single pulse of a nanosecond excimer KrF laser. It was shown that the hydrophobic behavior of the pristine sample became hydrophilic after laser treatment. XPS was employed to study the effect of laser treatment on surface stoichiometry of the ZnO epilayers. The results revealed the formation of oxygen vacancies, which are envisaged to control the observed hydrophilic behavior. Our AFM studies showed surface smoothing due to the coupling of the high energy laser beam with the surface. The importance of integration of c-axis ZnO with Si(111) substrates is emphasized using the paradigm of domain matching epitaxy on the c-YSZ[111] buffer platform along with their out-of-plane orientation, which leads to improvement of the performance of the solid-state devices. The observed ultrafast response and switching in photochemical characteristics provide new opportunities for application of ZnO in smart catalysts, sensors, membranes, DNA self-assembly and multifunctional devices.

Individual ZnO nanowires for photodetectors with wide response range from solar-blind ultraviolet to near-infrared modulated by bias voltage and illumination intensity

ZnO nanowires have relatively high sensitivity as ultraviolet (UV) photodetectors, while the bandgap of 3.37 eV is an important limitation for their applications in solar-blind UV (SBUV), visible (VIS) and near infrared (NIR) range. Besides UV response, in this study, we demonstrate the promising applications of individual undoped ZnO NWs as high performance SBUV-VIS-NIR broad-spectral-response photodetectors, strongly depended on applied bias voltage and illumination intensity. The dominant mechanism is attributed to the existence of surface states in nanostructured ZnO. At a negative bias voltage electrons can be injected into surface states from electrode, and moreover, under light illumination photogenerated electron-hole pairs can be separated efficiently by surface built-in electric field, resulting into a decrease of potential barrier height and depletion region width, and simultaneously accompanying a filling of oxygen vacancy and a rise of ZnO Fermi level.

Functionalization of single-wall carbon nanotubes with chromophores of opposite internal dipole orientation

We report the functionalization of carbon nanotubes with two azobenzene-based chromophores with large internal dipole moments and opposite dipole orientations. The molecules are attached to the nanotubes noncovalently via a pyrene tether. A combination of characterization techniques shows uniform molecular coverage on the nanotubes, with minimal aggregation of excess chromophores on the substrate. The large on/off ratios and the subthreshold swings of the nanotube-based field-effect transistors (FETs) are preserved after functionalization, and different shifts in threshold voltage are observed for each chromophore. Ab initio calculations verify the properties of the synthesized chromophores and indicate very small charge transfer, confirming a strong, noncovalent functionalization.

Alternative plasmonic materials: beyond gold and silver

Materials research plays a vital role in transforming breakthrough scientific ideas into next-generation technology. Similar to the way silicon revolutionized the microelectronics industry, the proper materials can greatly impact the field of plasmonics and metamaterials. Currently, research in plasmonics and metamaterials lacks good material building blocks in order to realize useful devices. Such devices suffer from many drawbacks arising from the undesirable properties of their material building blocks, especially metals. There are many materials, other than conventional metallic components such as gold and silver, that exhibit metallic properties and provide advantages in device performance, design flexibility, fabrication, integration, and tunability. This review explores different material classes for plasmonic and metamaterial applications, such as conventional semiconductors, transparent conducting oxides, perovskite oxides, metal nitrides, silicides, germanides, and 2D materials such as graphene. This review provides a summary of the recent developments in the search for better plasmonic materials and an outlook of further research directions.

Passivation of surface states in the ZnO nanowire with thermally evaporated copper phthalocyanine for hybrid photodetectors

The adsorption of O2/H2O molecules on the ZnO nanowire (NW) surface results in the long lifetime of photo-generated carriers and thus benefits ZnO NW-based ultraviolet photodetectors by suppressing the dark current and improving the photocurrent gain, but the slow adsorption process also leads to slow detector response time. Here we show that a thermally evaporated copper phthalocyanine film is effective in passivating surface trap states of ZnO NWs. As a result, the organic/inorganic hybrid photodetector devices exhibit simultaneously improved photosensitivity and response time. This work suggests that it could be an effective way in interfacial passivation using organic/inorganic hybrid structures.

Rectification behavior of PATP self-assembled on ZnO microrod arrays

A rectifying hybrid junction was fabricated by the self-assembly of 4-aminothiophenol (PATP) on well-aligned ZnO microrod arrays. Good rectification behavior was obtained from the device of Al/ZnO/PATP/Al. The electron transport at the ZnO/PATP interface was investigated systematically by experimental observation and theoretical simulation. X-ray photoelectron spectroscopy (XPS) analysis confirmed the strong binding between PATP and ZnO via S-Zn bonds. The effective energy barrier and ideality factor of the rectifying diode were estimated by the current-voltage (I-V) measurement and thermionic emission theory. The molecule dipole effect on work function was studied through energy band theory. Theoretical calculation results based on density functional theory (DFT) also indicated a significant dipole, caused by the anchoring effect of PATP, resulting in the changes of surface electronic characteristics of ZnO.

On the stabilization mechanisms of organic functional groups on ZnO surfaces

Density functional theory (DFT) calculations have been employed to investigate the interaction between ZnO-(101[combining macron]0) and (12[combining macron]10) surfaces and organic functional groups. We analyze the influence of the surface coverage on the geometries and binding energies under a dry environment. Our calculations show that coverages θ = 1 ML are favored under ligand-rich conditions and a dry environment. However, based on thermodynamic considerations we suggest that these ligands may not be stable against adsorption of liquid water and water vapor.

Water-repellent coatings prepared by modification of ZnO nanoparticles

Superhydrophobic coatings with a static water contact angle (WCA)>150° were prepared by modifying ZnO nanoparticles with stearic acid (ZnO@SA). ZnO nanoparticles of size ∼14nm were prepared by solution combustion method. X-ray diffraction (XRD) studies reveal that as prepared ZnO has hexagonal wurtzite structure whereas the modified coatings convert to zinc stearate. Field emission scanning electron micrographs (FE-SEM) show the dual morphology of the coatings exhibiting both particles and flakes. The flakes are highly fluffy in nature with voids and nanopores. Fourier transformed infrared (FTIR) spectrum shows the stearate ion co-ordinates with Zn(2+) in the bidentate form. The surface properties such as surface free energy (γ(p)) and work of adhesion (W) of the unmodified and modified ZnO coatings have been evaluated. The electron paramagnetic resonance (EPR) spectroscopy reveals that surface defects play a major role in the wetting behavior.

Functionalised zinc oxide nanowire gas sensors: Enhanced NO(2) gas sensor response by chemical modification of nanowire surfaces

Surface coating with an organic self-assembled monolayer (SAM) can enhance surface reactions or the absorption of specific gases and hence improve the response of a metal oxide (MOx) sensor toward particular target gases in the environment. In this study the effect of an adsorbed organic layer on the dynamic response of zinc oxide nanowire gas sensors was investigated. The effect of ZnO surface functionalisation by two different organic molecules, tris(hydroxymethyl)aminomethane (THMA) and dodecanethiol (DT), was studied. The response towards ammonia, nitrous oxide and nitrogen dioxide was investigated for three sensor configurations, namely pure ZnO nanowires, organic-coated ZnO nanowires and ZnO nanowires covered with a sparse layer of organic-coated ZnO nanoparticles. Exposure of the nanowire sensors to the oxidising gas NO(2) produced a significant and reproducible response. ZnO and THMA-coated ZnO nanowire sensors both readily detected NO(2) down to a concentration in the very low ppm range. Notably, the THMA-coated nanowires consistently displayed a small, enhanced response to NO(2) compared to uncoated ZnO nanowire sensors. At the lower concentration levels tested, ZnO nanowire sensors that were coated with THMA-capped ZnO nanoparticles were found to exhibit the greatest enhanced response. ΔR/R was two times greater than that for the as-prepared ZnO nanowire sensors. It is proposed that the ΔR/R enhancement in this case originates from the changes induced in the depletion-layer width of the ZnO nanoparticles that bridge ZnO nanowires resulting from THMA ligand binding to the surface of the particle coating. The heightened response and selectivity to the NO(2) target are positive results arising from the coating of these ZnO nanowire sensors with organic-SAM-functionalised ZnO nanoparticles.

Synthesis of nestlike ZnO hierarchically porous structures and analysis of their gas sensing properties

Nestlike 3D ZnO porous structures with size of 1.0-3.0 μm have been synthesized through annealing the zinc hydroxide carbonate precursor, which was obtained by a one-pot hydrothermal process with the assistance of glycine, Na(2)SO(4), and polyvinyl pyrrolidone (PVP). The nestlike 3D ZnO structures are built of 2D nanoflakes with the thickness of ca. 20 nm, which exhibit the nanoporous wormhole-like characteristic. The measured surface area is 36.4 m(2)g(-1) and the pore size is ca. 3-40 nm. The unique nestlike 3D ZnO porous structures provided large contacting surface area for electrons, oxygen and target gas molecules, and abundant channels for gas diffusion and mass transport. Gas sensing tests showed that the nestlike 3D ZnO porous structures exhibit excellent gas sensing performances such as high sensitivity and fast response and recovery speed, suggesting the potential applications as advanced gas sensing materials.

Formation of smooth, conformal molecular layers on ZnO surfaces via photochemical grafting

We have investigated the photochemical grafting of organic alkenes to atomically flat ZnO(10 ̅10) single crystals and ZnO nanorods as a way to produce functional molecule-semiconductor interfaces. Atomic force microscopy shows that photochemical grafting produces highly conformal, smooth molecular layers with no detectable changes in the underlying structure of the ZnO terraces or steps. X-ray photoelectron spectroscopy measurements show that grafting of a methyl ester-terminated alkene terminates near one monolayer, while alkenes bearing a trifluoroacetamide-protected amine form very smooth multilayers. Even with multilayers, it is possible to deprotect the amines and to link a second molecule to the surface with excellent efficiency and without significant loss of molecules from the surface. This demonstrates that the use of photochemical grafting, even in the case of multilayer formation, enables multistep chemical processes to be conducted on the ZnO surface. Photoresponse measurements demonstrate that functionalization of the surface does not affect the ability to induce field effects in the underlying ZnO, thereby suggesting that this approach to functionalization may be useful for applications in sensing and in hybrid organic-inorganic transistors and related devices.

Biomimetic micro∕nanostructured functional surfaces for microfluidic and tissue engineering applications

This paper reviews our work on the application of ultrafast pulsed laser micro∕nanoprocessing for the three-dimensional (3D) biomimetic modification of materials surfaces. It is shown that the artificial surfaces obtained by femtosecond-laser processing of Si in reactive gas atmosphere exhibit roughness at both micro- and nanoscales that mimics the hierarchical morphology of natural surfaces. Along with the spatial control of the topology, defining surface chemistry provides materials exhibiting notable wetting characteristics which are potentially useful for open microfluidic applications. Depending on the functional coating deposited on the laser patterned 3D structures, we can achieve artificial surfaces that are (a) of extremely low surface energy, thus water-repellent and self-cleaned, and (b) responsive, i.e., showing the ability to change their surface energy in response to different external stimuli such as light, electric field, and pH. Moreover, the behavior of different kinds of cells cultured on laser engineered substrates of various wettabilities was investigated. Experiments showed that it is possible to preferentially tune cell adhesion and growth through choosing proper combinations of surface topography and chemistry. It is concluded that the laser textured 3D micro∕nano-Si surfaces with controllability of roughness ratio and surface chemistry can advantageously serve as a novel means to elucidate the 3D cell-scaffold interactions for tissue engineering applications.

Self-powered, ultrafast, visible-blind UV detection and optical logical operation based on ZnO/GaN nanoscale p-n junctions

Ultrafast-response (20 μs) UV detectors, which are visible-blind and self-powered, in devices where an n-type ZnO nanowire partially lies on a p-type GaN film, are demonstrated. Moreover, a CdSe-nanowire red-light detector powered by a nanoscale ZnO/GaN photovoltaic cell is also demonstrated, which extends the device function to a selective multiwavelength photodetector and shows the function of an optical logical AND gate.

Structural transition from MgZnO nanowires to ultrathin nanowalls by surface separation: growth evolution and gas sensing properties

This paper reports a spontaneous method of controlling the growth mode from vertically arrayed ultra-slim MgZnO nanowires to nanowalls through the in-plane random motion of the seed crystals formed by surface phase separation. Seed crystals with a relatively Zn-rich phase were formed by the simultaneous injection of Mg and Zn and became strongly networked when the Zn/Mg flux ratio was increased at high temperatures, leading to the formation of MgZnO nanowalls on various conducting substrates. The hydrogen sensing performance of the MgZnO nanowalls with a two-dimensional network structure was superior to that of the one-dimensional MgZnO nanowires. Based on the microstructural characterizations, the growth procedure for the structural transition from MgZnO nanowires to nanowalls on the Si substrates was proposed.

Low energy electron point source microscopy: beyond imaging

Low energy electron point source (LEEPS) microscopy has the capability to record in-line holograms at very high magnifications with a fairly simple set-up. After the holograms are numerically reconstructed, structural features with the size of about 2 nm can be resolved. The achievement of an even higher resolution has been predicted. However, a number of obstacles are known to impede the realization of this goal, for example the presence of electric fields around the imaged object, electrostatic charging or radiation induced processes. This topical review gives an overview of the achievements as well as the difficulties in the efforts to shift the resolution limit of LEEPS microscopy towards the atomic level. A special emphasis is laid on the high sensitivity of low energy electrons to electrical fields, which limits the structural determination of the imaged objects. On the other hand, the investigation of the electrical field around objects of known structure is very useful for other tasks and LEEPS microscopy can be extended beyond the task of imaging. The determination of the electrical resistance of individual nanowires can be achieved by a proper analysis of the corresponding LEEPS micrographs. This conductivity imaging may be a very useful application for LEEPS microscopes.

Surface functionalization of zinc oxide by carboxyalkylphosphonic acid self-assembled monolayers

Two carboxyalkylphosphonic acids (HOOC(CH(2))(n)P(O)(OH)(2), n = 2 for 3-PPA and n = 9 for 10-PDA) have been deposited onto 1D zinc oxide (ZnO) nanowires and bare ZnO wafers to form stable self-assembled monolayers (SAMs). The samples were systematically characterized using wettability, atomic force microscopy (AFM), Fourier transform infrared spectroscopy (FT-IR), and X-ray photoelectron spectroscopy (XPS). 3-PPA was bound to the ZnO surfaces mainly through the CO(2)H headgroup, and 10-PDA formed self-assembled monolayers on the nanoscaled ZnO surface through the PO(3)H(2) headgroups. To verify the potential utilization of the functionalized surfaces in the construction of biosensors or bioelectronics, IgG (immunoglobulin G) protein immobilization through SAM bridging was demonstrated. This work expands the application of phosphonic acid-based surface functionalization on sensing and optoelectronic devices.

Photoresponse of a single poly(p-phenylene vinylene)-CdSe bulk-heterojunction submicron fiber

Bulk-heterojunction PPV (poly(p-phenylene vinylene))-CdSe submicron fibers have been prepared by introducing electrostatic interaction between thioglycolic acid capped CdSe nanocrystals and PPV precursor, subsequently with electrospinning and thermal conversion. The photoconductive device on a single PPV-CdSe fiber showed notable photoresponse, good response speed, wavelength-sensitivity, and reproducibility.

Enhanced UV photoresponse in nitrogen plasma ZnO nanotubes

The photoresponse behavior of one-dimensional ZnO nanowires (NWs) and nanotubes (NTs) grown on ITO-coated glass substrates via a wet-chemical route was investigated. The photoluminescence spectra exhibited a decrease in the deep-level intensity, indicating that the oxygen defects and impurities are occupied by the presence of N ions in the ZnO NT matrix after a nitrogen plasma treatment. I-V tests demonstrate an enhanced dark current (4.83 x 10(-7) A) after an extended plasma treatment of up to 900 s for ZnO NTs compared to that (0.571 x 10(-7) A) of NWs. Furthermore, the ZnO NTs show the highest reliable photoresponse, 20 times that of NWs under UV irradiation (325 nm) in air at room temperature. It is believed that nitrogen plasma ZnO nanotubes can potentially be useful in the designs of 1D ZnO-based solar cells and optoelectronic devices.

A nanoelectronic nose: a hybrid nanowire/carbon nanotube sensor array with integrated micromachined hotplates for sensitive gas discrimination

A novel hybrid chemical sensor array composed of individual In(2)O(3) nanowires, SnO(2) nanowires, ZnO nanowires, and single-walled carbon nanotubes with integrated micromachined hotplates for sensitive gas discrimination was demonstrated. Key features of our approach include the integration of nanowire and carbon nanotube sensors, precise control of the sensor temperature using the micromachined hotplates, and the use of principal component analysis for pattern recognition. This sensor array was exposed to important industrial gases such as hydrogen, ethanol and nitrogen dioxide at different concentrations and sensing temperatures, and an excellent selectivity was obtained to build up an interesting 'smell-print' library of these gases. Principal component analysis of the sensing results showed great discrimination of those three tested chemicals, and in-depth analysis revealed clear improvement of selectivity by the integration of carbon nanotube sensors. This nanoelectronic nose approach has great potential for detecting and discriminating between a wide variety of gases, including explosive ones and nerve agents.

Polymer functionalized ZnO nanobelts as oxygen sensors with a significant response enhancement

A plasma-polymerized acrylonitrile (PP-AN)/ZnO nanobelt (NB) nanosensor reveals a better oxygen-sensing response than a bare ZnO NB nanosensor due to the sorption nature of the polymer. With the aid of UV light, significant response enhancements of PP-AN/ZnO NB nanosensors at low temperature have been observed since the effects of oxygen desorption/adsorption in PP-AN on the electron depletion region in the ZnO are significant. The minimum sensitivity at 150 degrees C is 16.6 ppm. This work permits its feasibility in areas where it is impossible to work at higher temperatures since lowering the working temperature of the sensor can avoid the structural deterioration, causing instability in the response.

Tailoring the photoluminescence of ZnO nanowires using Au nanoparticles

Rational design of hybrid nanostructures through attaching nanowires with nanoparticles is an effective route to enhance the existing functionalities or to explore new ones. We carry out a systematic investigation on the photoluminescence of ZnO nanowire-Au nanoparticle hybrid nanostructures synthesized by attaching Au nanoparticles onto ZnO nanowires. Citrate-stabilized 40 nm Au nanoparticles effectively quench the green emission and enhance the UV emission of the ZnO nanowires, which is consistent with the wavelength-dependent generation of surface plasmon. The UV/green emission intensity ratio could be reversibly and reproducibly tailored by attaching/detaching Au nanoparticles. This enhancement of UV emission diminishes if the Au nanoparticles are coated with a polymer layer. We also find that the orange-red emission of the ZnO nanowires is related to the excess oxygen on the ZnO surface, and it is also tunable via annealing and surface modifications.

Photovoltaic properties of a ZnO nanorod array affected by ethanol and liquid-crystalline porphyrin

A vertically aligned array of ZnO nanorods, fabricated on conductive ITO substrate in aqueous solution, was characterized by scanning electron microscopy (SEM), x-ray diffraction (XRD), and UV-visible transmission spectroscopy. Surface photovoltage (SPV) techniques based on a lock-in amplifier and a Kelvin probe were both employed to study the photogenerated charges in the system. The effects of ethanol solvent and a liquid-crystalline porphyrin, [5-(para-dodecyloxy)phenyl-10,15,20-tri-phenyl] porphyrin (DPTPP), on the photovoltage enhancement in the ZnO nanorod array were studied via SPV comparison between different irradiation directions on the system. We demonstrate that the ethanol adsorption could induce the space charge region to expand towards the ZnO/ITO interface. In the absence of ethanol, the ZnO nanorod array with the DPTPP adsorption showed enhanced SPV with reduced attenuation rate of photogenerated charge carriers. We found that the separation of photogenerated charges could be further improved by coating the surface with DPTPP and ethanol together. Furthermore, the SPV spectra patterns of the composite system with opposite incident-light directions reveal that the DPTPP molecules adsorbed just at the surface of ZnO nanorods adopt a more monomeric alignment in contrast to the aggregative state in the DPTPP bulk.

Direct synthesis of ZnO nanowire arrays on Zn foil by a simple thermal evaporation process

ZnO nanowire arrays were synthesized on zinc foil by a simple thermal evaporation process at relatively low temperature. Morphology and size controlled synthesis of the ZnO nanostructures was achieved by variation of the synthesis temperature, reaction time and the surface roughness of the substrate. A gas-solid and self-catalytic liquid-solid mechanism is proposed for the growth of nanowires at different temperatures. High-resolution transmission electron microscopy (HRTEM) showed that the as-grown nanowires were of single crystal hexagonal wurtzite structure, growing along the [101] direction. Photoluminescence exhibited strong UV emission at ∼382 nm and a broad green emission at ∼513 nm with 325 nm excitation. Raman spectroscopy revealed a phonon confinement effect when compared with results from bulk ZnO. The nanowire arrays also exhibited a field emission property.