All Printable ZnO Nanowire Photodetectors with Ultra-High Detectivity

High performance photodetectors are critical for high speed optical communication and environmental sensing. And flexible photodetectors can be used for a wide range of portable/wearable applications. Low dimensionality of semiconductor nanowires (SNWs) renders them intriguing physical and chemical properties, such as resonant light absorption, carrier confinement induced band-gap tunability, rich of surface state and large surface area¹, excellent mechanical flexibility^2-4, etc. These remarkable properties offer the tantalizing possibility of using SNWs for various unique functions and/or high performance electronic and optoelectronic devices, e.g. high sensitivity gas/chemical sensing, in addition to their favorable dimensions for device miniaturization. Interestingly, recently studies also revealed that the surface states on SNW results in appreciable energy band bending in radial direction of SNWs, which could leads to separation of photo-generated electrons and holes in the radial direction thus dramatically prolongs carrier life-time and leads to high photocurrent gain up to ~10⁸. Nevertheless, the majority of nanowire photodetection studies have been based on single crystalline nanowires which can only implement band edge modulation along radial direction. In these cases the energy bands, i. e. conduction and valance band edge, are still flat along the axial direction, which provide a fast track for carrier transport when external electric field is applied. This inevitably results in high dark current due to the intrinsic carrier concentration, and limits device performance such as Noise Equivalent Power (NEP), Detectivity (D*), Linear Dynamic Range (LDR), etc.

In this work, we demonstrate photodetection of unique parallel arrayed ZnO granular NWs (GNWs) with not only high internal gain and responsivity at low bias, but also ultra-high detectivity due to the significantly suppressed dark current. The key to the ultra-high detectivity rests in the modulated energy band edge along the axial direction of the nanowires due to the existence of the grain boundaries. These grain boundaries results in multiple barriers for axial electron transport blocking dark current effectively. Meanwhile, barrier height reduction upon illumination leads to drastically increased photocurrent. This gain mechanism is distinctively different from that of single crystal SNW photodetectors. As the result, the GNWs show detectivity as high as 3.3×10¹⁷Jones with capability of detecting ~2.3 photon/sec flux on each nanowires statistically, showing their great potency as high performance photosensors. In addition to the appealing photodetection performance, the developed all-printable fabrication process is highly facile and versatile. Particularly, parallel arrays of GNWs with controllable spacing are printed on flexible substrates with a programmable near-field electrospinning setup followed by proper heat treatment. Thereafter the metal electrodes are deposited on the GNWs with ink-jet printing technique leading to formation of flexible GNW photodetectors. It is worth noting that this process is much more scalable and cost-effective as compared to the conventional bottom-up and top-down approaches for nanomaterial growth and device fabrication. Therefore it is highly attractive for large scale manufacturing of next generation high performance flexible electronic devices based on nanomaterials. In addition, the discovered rationale in this work can be utilized as guidelines to design high performance photodetectors with other nanomaterial systems as well. Meanwhile, the developed fabrication scheme opens up possibility for future flexible and high performance integrated optoelectronic sensor circuitry.

REFERENCES AND NOTES

1. Pan, X., Liu, X., Bermak, A. & Fan, Z. Self-Gating Effect Induced Large Performance Improvement of ZnO Nanocomb Gas Sensors. ACS Nano (2013).

2. Liu, X., Long, Y., Liao, L., Duan, X. & Fan, Z. Large-scale integration of semiconductor nanowires for high-performance flexible electronics. ACS Nano 6, 1888-1900 (2012).

3. Fan, Z. Y. et al. Three-dimensional nanopillar-array photovoltaics on low-cost and flexible substrate. Nat. Mater. 8, 648-653 (2009).

4. Fan, Z. Y. et al. Toward the Development of Printable Nanowire Electronics and Sensors. Adv. Mater. 21, 3730-3743 (2009).