ZnO Nanobridge Research Articles

Toward facile broadband photodetectors based on self-assembled ZnO nanobridge/rubrene heterointerface

ZnO nanowire photodetectors have attracted much attention due to their excellent optoelectronic performance. However, operating speed remains a challenge, and scalability is also impeded by uncontrolled transfer methods and sophisticated fabrication process. In this paper, we have fabricated an excellent ZnO nanobridge ultraviolet photodetector array by using a simple one-step method. The faster photoresponse speed and a broader response wavelength (from UV to visible range) have been achieved by constructing a type-II ZnO/rubrene heterointerface. Performance enhancement is believed to arise from the well-matching band alignment and highly efficient separation of photogenerated electron-hole pairs at the heterointerface. Our strategy provides a simple and promising route to develop cost-effective and highly sensitive UV–vis photodetectors.

Self-Bridged ZnO Nanowires through Simple Annealing Process and Its Humidity Sensing Properties

One-dimensional (1D) nanomaterials, including nanowires, nanotubes, nanobelts, etc., are an important class of nanomaterials due to their uniquely anisotropic structures, which make them very promising as the functional materials for energy storage, photonics, catalysis, and nano-scale devices fabrication. Among which, 1D metal oxide nanomaterials are considered as some of the most fascinating functional materials and had been widely exploited [1-3]. On the other hand, humidity sensing based on metal oxides attracted a lot of attentions in meterology, health science, food science, and agriculture. In the past years, lots of detection method have been explored. Nowadays, capacitive, resistive detection methods are used. To improve the sensitivity, selectivity and stability at various condition, enormous efforts have been put on humidity sensor based on nanostructured materials. Therefore, it is of significance to develop the controlled synthesis of 1D metal oxide nanomaterials, considering the above-mentioned two aspects. ZnO nanowires demonstrate promising properties as sensing elements and widely used for UV sensing, humidity sensing, and gas sensing [4, 5]. Most work on growth of ZnO nanowires using vapor-liquid-solid (VLS) techniques and vapor-solid (VS) techniques. The synthesized nanowires are rearranged perpendicular to its substrate using “pick and place” technique. However, such a technique requires lots of time and efforts. Although enormous efforts have been put on massive production of ZnO nanowires, its directional growth between microelectrodes for the nano-device fabrication was still full of challenges [6]. In this study, ZnO nanowires were synthesized by controlled annealing process by which the interconnections between microelectrodes were made. Before annealing process, electrochemical deposition was carried out in the mixed electrolyte, containing 0.1 M ZnCl2and 1 M KCl. For verification of ZnO nanowires as a sensor material, scanning electron microscope (SEM), and X-ray diffraction (XRD) were carried out to characterized their morphology and microstructures, respectively. Also, electrical properties of ZnO nanowires were measured by semiconductor analyzer in the range from -1 V to 1 V. The characteristics of humidity sensor were confirmed with the variation of the current at 1 V based on relative humidity controlled with mass flow controller by combining dry gas and wet gas. REFERENCES [1] Qin Kuang, Changshi Lao, Zhong Lin Wang, Zhaoxiong Xie and Lansun Zheng, “High-Sensitivity humidity sensor based on a single SnO2nanowire,” J. AM. Chem. SOC. 2007, 129, 6070-6071. [2] Ming Zhuo, Yuejiao Chen, Jia Sun, Haiming Zhang, Di Guo, Haonan Zhang, Qiuhong Li, Qaihong Wang and Qing Wan, “Humidity sensing properties of a single Sb doped SnO2nanowire field effect transistor,” Sens. Actuators, B 2013, 189, 78-83. [3] Zhenyu Wang, Yan Xiao, Xiaobiao Cui, Pengfei Cheng, Biao Wang, Yuan Gao, Xiaowei Li, Tianlin Yang, Tong Zhang and Geyu Lu, “Humidity-Sensing properties of urchinlike CuO nanostructures modified by reduced graphene oxide,” ACS Appl. Mater. Interfaces 2014, 6, 3888-3895. [4] Sanghwa Yoon, Jae-Hong Lim and Bongyoung Yoo, “Oxygen re-adsorption of a single ZnO nanobridge by joule heating under ultraviolet illumination,” Appl. Phys. Express 2012, 5, 105003. [5] Nai-Feng Hsu and Tien-Kan Chung, “A rapid synthesis/growth process producing massive ZnO nanowires for humidity and gas sensing,” Appl. Phys. A 2014, 116, 1261-1269. [6] J.B.K. Law, C. B. Boothroyd and J. T. L. Thong, “Site-specific growth of ZnO nanowires from patterned Zn via compatible semiconductor processing,” J. Crystal growth 2008, 310, 2485-2492

Impact of Parylene-A Encapsulation on ZnO Nanobridge Sensors and Sensitivity Enhancement via Continuous Ultraviolet Illumination

The impact of parylene-A encapsulation and the effect of continuous ultraviolet (UV) exposure on ZnO nanobridge sensor response are investigated. ZnO nanowire (NW) devices are fabricated using a novel method that involves selective growth of ZnO nanobridges between lithographically defined pads of carbonized photoresist (C-PR). We find that a thin coating of parylene-A effectively attenuates the response of NW devices to O2, H2O vapor, and UV illumination. The accessibility of the amine group on parylene-A for chemical functionalization is verified by transforming the amine groups on the surface of the parylene-A coating into aromatic imine groups, followed by UV–Vis absorption. Our results suggest that, in addition to modulating environmental sensitivity and providing protection of the ZnO NWs for liquid- and vapor-phase sensing, the parylene-A encapsulation may also serve as an activation layer for further specific functionalization targeting selective sensing. We also found that the sensitivity and response time of ZnO nanobridge devices to O2 are dramatically improved by continuously exposing the nanobridge devices to UV illumination. Finally, we show that the C-PR directed growth method can also be used to isolate free-standing NW carpet.

Functionalization and Environmental Stabilization of ZnO Nanobridge Sensors Fabricated using Carbonized Photoresist

ABSTRACTZnO nanobridge sensors were selectively grown and electrically connected using lithographically patterned carbonized photoresist (C-PR). As ZnO is known to dissolve in deionized (DI) water, parylene-A was investigated as a moisture barrier and potential functionalization platform. A chemical vapor deposition (CVD) process for parylene-A was developed and it was demonstrated that parylene-A coated ZnO NWs do not show any signs of dissolution after 24 hours in an aqueous solution. The impact of the parylene-A coating on the electrical performance and sensitivity of ZnO nanobridge devices was investigated. A comparison of UV, O2, and H2O sensitivity between uncoated and coated devices revealed that a thin coating of parylene-A attenuated all of these responses, suggesting the ability to modulate environmental sensitivity using this non-covalently bound polymer passivation layer.

Synthesis, functionalization, and environmental stabilization of ZnO nanobridge transducers for gas and liquid-phase sensing

Three methods of functionalizing ZnO NW surfaces with biotin were demonstrated. Biotinylated ZnO NWs were found to dissolve during exposure to deionized (DI) water, so a chemical vapor deposition (CVD) process was developed for parylene-A, a common moisture barrier with an amine group which allows further functionalization. Parylene-A coated ZnO NWs were found to be resistant to dissolution. Electrical measurements on parylene-A coated nanobridge devices showed normal operation with higher dark current and an attenuated response to UV and O 2, indicating the ability to modulate environmental sensitivity. This work demonstrates the novel use of parylene-A coatings as an encapsulation layer as well as a potential starting platform for general functionalization of ZnO NW devices for selective sensing.

ZnO nanobridge devices fabricated using carbonized photoresist

Despite high interest for novel device applications, alignment and electrical integration of nanowires to lithographically defined features remains a challenge. In this work, ZnO nanowire devices were fabricated using a novel carbonized photoresist method in which photoresist is lithographically patterned, carbonized at elevated temperature, and then used to selectively seed growth of ZnO nanobridges between opposing carbonized photoresist electrodes. The pick and place method is avoided and selective growth of nanobridge structures is achieved without the use of metal catalysts or inorganic seed layers. Growth and electrical connection take place simultaneously. Electrical characterization of the electrical contact between the carbonized photoresist electrodes and the ZnO nanobridges is performed and operation of nanobridge devices as bottom gate three terminal field effect devices is demonstrated.

Directed integration of ZnO nanobridge sensors using photolithographically patternedcarbonized photoresist

A method for achieving large area integration of nanowires into electrically accessibledevice structures remains a major challenge. We have achieved directed growth andintegration of ZnO nanobridge devices using photolithographically patterned carbonizedphotoresist and vapor transport. This carbonized photoresist method avoids the use ofmetal catalysts, seed layers, and pick and place processes. Growth and electrical connectiontake place simultaneously for many devices. Electrical measurements on carbonizedphotoresist/ZnO nanobridge/carbonized photoresist structures configured as three-terminalfield effect devices indicate bottom gate modulation of the conductivity of the n-type ZnOchannel. Nanobridge devices were found to perform well as ultraviolet and gas sensors, andwere characterized as regards ultraviolet light pulsing, oxygen concentration,and humidity. The sensitivity of the three-terminal nanobridge sensors to UVlight and oxygen was enhanced by application of a negative bottom gate voltage.

ZnO Nanobridge Array UV Photodetectors

This study describes the fabrication of ultraviolet photodetectors with laterally aligned ZnO nanobridge arrays. These nanobridge arrays grow upward in the face-to-face direction, thereby forming biaxial compressive stress where nanobridges intersect. Compared with conventional thin-film photodetectors, the nanobridge devices markedly enhance photosensitivity and blue shift (30 nm) of the spectral response. These phenomena are caused by surface effects of the ZnO nanobridge and strain-induced polarization effects, leading to band structure change. Nanobridge devices are a promising alternative for transforming advanced optoelectronic integration circuits with a 1D structure into miniaturized devices.

Photoresponses of ZnO nanobridge devices fabricated using a single-step thermal evaporation method

We have recently reported fabrication of ZnO nanobridge devices using a single-step thermal evaporation method. In this fabrication process, we completely eliminated the need of either any metal catalysts or a ZnO seed layer to synthesize the ZnO nanobridges. As initially postulated, the morphology of an anisotropic crystalline substrate alone defines the growth region to prevent the random growth of ZnO nanowires on the substrate. The photoresponses of a ZnO nanobridge device measured under the UV illumination ( λ = 365 nm) have indicated distinctively higher photosensitivity compared with a networked ZnO nanowire device fabricated by using a conventional photolithography technique. This result confirms that the single-step process not only provides a simple and a cost effective way to integrate self-assembled nanodevices comprised of individual and/or multiple ZnO nanobridges with conventional circuits without using e-beam lithography techniques and/or additional costly deposition processes but also allows one to fabricate devices with higher photosensitivity.

Direct Formation of Catalyst-Free ZnO Nanobridge Devices on an Etched Si Substrate Using a Thermal Evaporation Method

Well-aligned single crystalline ZnO nanobridges have been synthesized selectively across the prefabricated electrodes on silicon substrates by a single-step thermal evaporation method without using any metal catalysts or a predeposited ZnO seed layer that was a prerequisite for such synthesis. The growth region was self-defined by the anisotropic surface of the silicon substrate as initially postulated. Careful control of the reaction time and the substrate temperature allows the nanobridges to form almost exclusively across the electrodes. The photoresponses as well as the current-voltage characteristics of the device confirm that this single-step process indeed offers a simple and a cost-effective way to integrate self-assembled nanodevices based on individual and/or a large number of ZnO nanowires with conventional circuits without using e-beam lithography techniques and/or additional costly deposition processes.

Selective growth and directed integration of ZnO nanobridge devices on si substrates without a metal catalyst using a ZnO seed layer

Directed assembly and integration of ZnO nanobridges (NBs) into working devices on Si substrates was achieved. Metal catalysts were not used, and the “harvest and disperse” method of nanorod (NR) integration was avoided. High-quality ZnO NRs were grown via a vapor-solid method selectively on a patterned thin-film ZnO seed layer. ZnO NRs exhibited a single-crystalline structure with c-axis preferred orientation, were aligned roughly perpendicular to the surface, and showed a strong intrinsic near-band edge photoluminescence peak at 380 nm with no detectable visible photoluminescence. These ZnO NRs were grown to span gaps between separated electrodes, terminate on an opposing surface, and effectively form electrically accessible vertical or horizontal ZnO NB devices. Operation of the directly grown horizontal ZnO NB devices as gas and UV sensors was performed, demonstrating a significant step toward practical large-scale integration of nanodevices into Si microelectronics.

Directed integration of ZnO nanobridge devices on a Si substrate

We demonstrate the directed assembly and integration of ZnO nanobridges into working devices on silicon-on-insulator substrates. The “pick and place” method of nanowire integration is avoided and metal catalysts are not used. ZnO nanowires (NWs) were grown selectively via a vapor-solid method using a patterned ZnO thin-film seed layer that was deposited on Si trench sidewalls via atomic layer deposition. ZnO NWs grew to span the trench and self-terminate on the opposing surface, effectively forming electrically accessible horizontal ZnO nanobridge devices. Vertical bridge devices were also constructed using undercut islands. Directly grown horizontal ZnO nanobridge devices were operated as gas and UV sensors, demonstrating that this method represents a significant step towards practical large-scale integration of nanodevices into Si microelectronics.