Junjie Qi

Scanning Probe Study on the Piezotronic Effect in ZnO Nanomaterials and Nanodevices

ZnO nanomaterials with their unique semiconducting and piezoelectric coupled properties have become promising materials for applications in piezotronic devices including nanogenerators, piezoelectric field effect transistors, and diodes. This article will mainly introduce the research progress on piezotronic properties of ZnO nanomaterials investigated by scanning probe microscopy (SPM) and ZnO-based prototype piezotronic nanodevices built in virtue of SPM, including piezoelectric field effect transistors, piezoelectric diodes, and strain sensors. Additionally, nanodamage and nanofailure of ZnO materials and their relevant piezotronic nanodevices will be critically discussed in their safe service in future nanoelectromechanical system (NEMS) engineering.

Self-powered ultraviolet photodetector based on a single Sb-doped ZnO nanobelt

We report a self-powered ultraviolet photodetector based on a single Sb-doped ZnO nanobelt bridging an Ohmic contact and a Schottky contact. The photoresponse sensitivity and the response time of the fabricated device are as high as 2200% and less than 100 ms, respectively. The performance of the device dramatically degrades as the Sb-doping concentration decreases in the ZnO nanobelt. The possible mechanisms have been proposed and discussed.

Size Dependence of Dielectric Constant in a Single Pencil-Like ZnO Nanowire

Scanning conductance microscopy (SCM) is used to measure the dielectric constant of a single pencil-like zinc oxide (ZnO) nanowire with the diameters ranging from 85 to 285 nm. As the diameter decreases, the dielectric constant of ZnO nanowire is found to decrease from 6.4 to 2.7, which is much smaller than that of the bulk ZnO of 8.66. A core-shell composite nanowire model in terms of the surface dielectric weakening effect is proposed to explore the origin of the size dependence of dielectric constant, and the experimental results are well explained.

Piezoelectric effect in chemical vapour deposition-grown atomic-monolayer triangular molybdenum disulfide piezotronics

High-performance piezoelectricity in monolayer semiconducting transition metal dichalcogenides is highly desirable for the development of nanosensors, piezotronics and photo-piezotransistors. Here we report the experimental study of the theoretically predicted piezoelectric effect in triangle monolayer MoS2 devices under isotropic mechanical deformation. The experimental observation indicates that the conductivity of MoS2 devices can be actively modulated by the piezoelectric charge polarization-induced built-in electric field under strain variation. These polarization charges alter the Schottky barrier height on both contacts, resulting in a barrier height increase with increasing compressive strain and decrease with increasing tensile strain. The underlying mechanism of strain-induced in-plane charge polarization is proposed and discussed using energy band diagrams. In addition, a new type of MoS2 strain/force sensor built using a monolayer MoS2 triangle is also demonstrated. Our results provide evidence for strain-gating monolayer MoS2 piezotronics, a promising avenue for achieving augmented functionalities in next-generation electronic and mechanical–electronic nanodevices.

Controllable fabrication and electromechanical characterization of single crystalline Sb-doped ZnO nanobelts

We report the fabrication of the high-quality Sb-doped ZnO nanobelts by using a simple chemical vapor deposition method. The nanobelts consist of single-crystalline wurtzite ZnO crystal and grow along [011¯2] direction. An electromechanical system is constructed to explore the transverse electrical properties of a single nanobelt under the different applied loading forces. The I-V results indicate that a little barrier exists in between the nanobelt and the atomic force microscopy tip. An almost linear relationship between the force and the resistance was found at small deformation regions, which demonstrates that the nanobelts have potential applications as force/pressure sensor for measuring the nano-Newton forces.

Piezotronic interface engineering on ZnO/Au-based Schottky junction for enhanced photoresponse of a flexible self-powered UV detector.

Exploiting piezoelectric effect to engineer material interface has been confirmed as a promising way to optimize the performance of optoelectronic devices. Here, by using this effect, we have greatly improved the photoresponse of the fabricated ZnO/Au Schottky junction based self-powered UV detector. A 440% augment of photocurrent, together with 5× increased sensitivity, was obtained when the device was subjected to a 0.580% tensile strain. The enhancement can be attributed to the facility separation and extraction of photoexcites due to the formation of the stronger and expanding built-in field, which is a result of charge redistribution induced by piezoelectric polarization at the ZnO/Au interface. This study not only can strengthen the understanding of piezoelectric effects on energy devices but also can be extended to boost performances of optoelectronic devices made of piezoelectric semiconductor materials.

High-performance piezoelectric gate diode of a single polar-surface dominated ZnO nanobelt

We report a piezoelectric gated diode that is composed of a single ZnO nanobelt with +/- (0001) polar surfaces being connected to an indium tin oxide (ITO) electrode and an atomic force microscopy (AFM) tip, respectively. The electrical transport is controlled by both the Schottky barrier and the piezoelectric barrier modulated by the applied forces. The diode exhibits a high ON/OFF current ratio (up to 1.6 x 10(4)) and a low threshold force of about 180 nN at 4.5 V bias. The electrical hysteresis is suggested to be attributed to be carrier trapping in the piezoelectric electric field.

Doping and defects in the formation of single-crystal ZnO nanodisks

High purity growth of polar surface dominated ZnO nanodisks was fabricated by introducing In ions in the raw material by thermal evaporation process without a catalyst. The nature of the sharp-contrast lines in the disks was investigated. The results suggested that the existence of sharp-contrast lines is due to the local segregation of In. Defects were initiated by segregation of the doping element of indium, which reduced the surface energy of ZnO (0001) leading to the fastest growth of the nanodisks along ⟨011¯0⟩. The preferred growth along ⟨011¯0⟩ is considered to maximize the effect of the piezoelectricity.

Flexible piezoresistive strain sensor based on single Sb-doped ZnO nanobelts

Using a two-end bonded Sb-doped ZnO nanobelt on a flexible polystyrene substrate, the decrease of the resistance with increasing compressed strains in the nanobelt has been observed, which is suggested to be attributed to the piezoresistance effect. The longitudinal piezoresistance coefficient of the Sb-doped ZnO nanobelt is about 350. On the basis of this finding, we made a flexible piezoresistive strain sensor in a signature pen, which can be used to detect the corresponding compressed strains when the characters are recorded.

Electrical breakdown of ZnO nanowires in metal-semiconductor-metal structure

We investigated the stability of ZnO nanowires in a metal-semiconductor-metal structure by applying a longitudinal electric field inside a scanning electron microscope equipped with manipulators. The electrical transport was well simulated by the thermionic-field-emission model and the failure of single crystalline ZnO nanowires was directly observed when the applied electric field reached the break point, an electric field intensity of ∼106 V/m. The recrystallization of ZnO nanowires from single crystalline to polycrystalline pearl-like structure in the failure process was also investigated. Experimental results indicated that the failure is attributed to a joint effect of high electric field and Joule heating.