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Rapid Fabrication Technique for Interpenetrated ZnO Nanotetrapod Networks for Fast UV Sensors

Dawit Gedamu , Ingo Paulowicz , Sören Kaps , Oleg Lupan , Sebastian Wille , Galina Haidarschin , Yogendra Kumar Mishra ,* and Rainer Adelung *

Recent developments on the synthesis of networked nanostruc-tures have added new amplitudes for designing multifunc-tional devices, including sensors and UV photodetectors. The importance of UV detectors is growing because of the demand for wide range of environmental, military, industrial, fl ame sensing, water sterilization, and early missile plume detection applications. [ 1 ] With their high performances, but own draw-backs, Si-, GaN- or III–V-compound-based UV detectors are available on the market. Although the Si-based photo-devices are known for quick responses compared with ZnO-based UV detectors, they exhibit severe limitations, such as poor selectivity towards visible and infrared (IR) light, which demands com-plex fi lters, ultra-high vacuum condition, and high voltage. [ 2 ] By contrast, for ZnO-based UV detectors, these types of limita-tions are easily ruled out because of their wide and direct band-gaps and ability to operate in harsh environmental conditions. In this work, we demonstrate two novel fl ame transport syn-thesis (FTS) methods, namely, burner fl ame transport synthesis (B-FTS) and crucible fl ame transport synthesis (C-FTS) for fab-rication of the ZnO nano-microstructure-based UV photodetec-tors. The FTS approach utilizes Zn microparticles in contrast to many conventional fl ame synthesis methods that are based on precursor gases. Both of the techniques allow facile and cost-effective growth of ZnO nano-microstructures, bridging the 2–10 μ m size gaps between two Au contacts on pre-patterned chips. High crystalline qualities of the fabricated nano-micro-structures grown by rapid B-FTS and C-FTS approaches were confi rmed by XRD and Raman studies. The SEM investigations revealed interpenetrating nanojunctions between the bridging ZnO nano-microstructures grown on the gaps between two

Au contacts. The B-FTS approach exhibits the unique feature of ultra-rapid growth of ZnO nanotetrapods within few milli-seconds and simultaneously in situ bridging electrical contacts. These bridging nanotetrapods were directly integrated on a chip and demonstrated signifi cantly improved performances as a UV photodetector. Comparison of the UV photodetectors per-formances built from interpenetrating ZnO nano-microstruc-tures fabricated by B-FTS and C-FTS techniques are presented. Fastest response/recovery time constant (≈32 ms) under 365 nm UV light irradiation of B-FTS-made photodetectors (on/off ratio ≈4.5 × 10 3 at 2.4 V) is reported. Different type of nanojunctions formed between neighbor nanowires or nanotet-rapods (with ‘arm’ thickness <50 nm) could be the reason for such improved characteristics. The role of nanojunctions in fast UV photodetectors from networked ZnO nanowires and nano-tetrapods is discussed. On the basis of the rapid B-FTS fabri-cation process and fast UV photodetection capabilities, such networked ZnO nanotetrapods can be potential candidates for various nanosensor applications.

Metal oxide nanostructures, such as nanorods, nano wires, and nanotetrapods, have gained a lot of research interest for multifunctional device applications owing to their elec-trical, optical, and mechanical properties. [ 3–6 ] With wide direct bandgap (3.37 eV) and large exciton binding energy (≈60 meV) at room temperature, ZnO nanostructures are one of the most investigated materials for different nanotechnological appli-cations. [ 4,6–9 ] The hexagonal wurtzite crystal structure with Zn 2+ - and O 2− -terminated polar surfaces along c-axis offer easy growth of ZnO structures, such as nanorods, nanowires, and nanotetrapods, in a facile manner. [ 3,10 ] It is well known that ZnO nanostructures are UV-light-sensitive and have been intensively investigated for photodetector applications. [ 11,12 ] ZnO nanowires have been treated under different conditions for understanding the fundamental process and enhancing the response to UV light. [ 13–15 ] Chai et al. [ 14 ] investigated UV photo-detection capabilities of an individual naturally self-assembled crossed ZnO nanorods. Chow et al. [ 15 ] reported UV photode-tection capabilities of an individual ZnO tetrapod and crossed ZnO nanorod and explained its photoresponse mechanism. The photocurrent response of highly crystalline ZnO nanowire arrays synthesized on polymer and fl exible fi ber substrates have been studied under different UV illumination intensities. [ 16 ] Pham et al. [ 17 ] reported that UV illumination can signifi cantly engineer the piezoelectric potential. Integrated ZnO nanowires in triple junction solar cells as UV detector have been reported to improve the current density of the solar cell under illumina-tion. [ 18 ] Bai et al. have designed a UV sensor by parallel con-nected ZnO nanowires on rigid and fl exible substrates using DOI: 10.1002/adma.201304363

D. Gedamu, I. Paulowicz, S. Kaps, Dr. O. Lupan, Dr. S. Wille, G. Haidarschin, Dr. Y. K. Mishra, Prof. R. Adelung Functional Nanomaterials, Institute for Materials Science University of KielKaiser Strasse 2 , D-24143 Kiel , Germany E-mail: ykm@tf.uni-kiel.de ; ra@tf.uni-kiel.de Associate Professor Dr. O. LupanDepartment of Microelectronics and Semiconductors Devices Technical University of Moldova 168 Stefan cel Mare Blvd. , MD-2004 Chisinau , Republic of Moldova Dr. S. WilleUniversitätsklinikum Schleswig-Holstein – Campus KielKlinik für Zahnärztliche Prothetik Propädeutik und Werkstoffkunde Arnold-Heller-Strasse 3 – Haus 26 , D-24105 Kiel , Germany

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ON contact printing method and reported that this sensor exhibits

high on/off ratio (≈12 × 10 4 @ 3 V and 14.1 mA) and fast recovery time (≈0.9 s). [ 19 ] ZnO nanowire sheets were embedded inside polydimethylsiloxane (PDMS) polymer for detection of UV light, and it was observed that PDMS coating reduces the photoresponse speed as well as prolongs the decay times almost by a factor of 4 as compared to that in air. [ 20 ] Ultrafast photo-detectors have been fabricated by using ZnO nanowire–GaN and ZnS–ZnO biaxial nanobelts, and it has been observed that biaxial nanobelt photodetectors exhibit a wide range of photore-sponses. [ 21 ] Coating of gold nanoparticles on the surface of ZnO nanorods [ 22 ] can signifi cantly enhance the photoconductivity of UV sensor device.

Various designs of photodetectors have been investigated to enhance the photodetection response and minimize the decay times and basic understanding about the underlying funda-mental phenomenon has also been developed, [ 23 ] but still there are a lot of open issues about the integration of nanostructures in a UV photodetector and their response/recovery times. Numerous physical and chemical processes have been devel-oped to grow different metal oxide nanostructures for UV pho-todetector applications. Vapor-liquid-solid (VLS) [ 24,25 ] processes are one of the most often used processes for growing quasi-1D nanorods and nanowires. VLS approaches offer controlled growth of ZnO Q1D nanostructures, but require specifi c equip-ment under optimum conditions, and large-scale fabrication is still a challenging task. Using chemical and electrochemical depositions various ZnO nanostructures, including tetrapods, have been successfully synthesized. [ 14,26 ] Chemical vapor depo-sition (CVD) [ 27 ] also offers the possibilities of growing different type of ZnO nanostructures for sensors and photodetectors. Recent introduced plasma and plasma-enhanced CVD pro-cesses also allow versatile fabrication of various metal oxide and other wide bandgap semiconductor nanostructures with interesting electronic and photonic properties. [ 28–30 ] Although the synthesis of ZnO nano-microstructures using VLS, CVD, or other techniques has been extensively studied and optimized in the past few years, very few reports on direct integration of such nanostructures on a chip and in devices. Direct integra-tion leads to a tight interface between nanostructure and con-tact pad material, while separately grown nanostructures dis-persed on contacts form gaps owing to the different inherent roughnesses of contact and nanostructure surfaces. Alongside the techniques used, the quality of synthesized nanostruc-tures is very important from the device aspect as the crystal-line nature and surface roughness of nanostructures also have a very important role in device performance. [ 31 ] Therefore to build a device, nanowires of desired quality are required to be synthesized by an appropriate technique followed by integra-tion of these quasi-1D nanostructures between the electrical contacts into the chip; this is one of the main challenges that hinder their wide application. To overcome some of these limitations, Adelung et al. [ 32 ] introduced the thin fi lm fracture approach for a direct fabrication of nanowires in a microscopic gap on chip with following integration in a device. Several other groups later on also demonstrated a direct fabrication of nanowire bridges on the chip for sensor and photodetector applications. [ 33 ] In another approach, Lupan and co-workers [ 34,35 ] made photodetectors using in situ lift-out technique, in which

a single ZnO nanorod/nanowire was picked up and mounted directly on the chip inside a focused ion beam system. Most of the methods described above use either sophisticated equip-ments/techniques to grow or exhibit diffi culty to integrate nanostructures into chip to build appropriate nanodevices at lower cost. But because of a large number of applications ranging from devices to biomedical engineering, material syn-thesis community is still looking for simple and cost-effective techniques that facilitate the effi cient and large-scale produc-tion of these nano-microstructures and their direct integration into the chip. Thus, fl ame transport-based techniques (B-FTS and C-FTS) have shown enormous potential to overcome with the aforementioned issues. [ 36,37 ]

In our work, the FTS techniques allowed us rapid synthesis and direct integration of networked nanostructures by in situ bridging the electrical contacts on chips for sensor device appli-cations. A fast response/recovery time and high I UV / I dark ratio for a UV photodetector made from B-FTS nanotetrapods net-work on a chip are demonstrated. Morphological and struc-tural evolutions of the B-FTS and C-FTS nanostructures are also investigated in detail to understand the photoconduction mechanism.

The direct growth of ZnO nanowires and nanotetrapods that are bridging the gaps of 2–10 μ m between two Au contacts on a pre-patterned chip ( Figure 1 a,b) using two variants of fl ame-transport-based techniques for photodetector applications, are schematically demonstrated in Figure 1 c,d. The basic concept of device structure is shown in Figure 1 a. Microstructured gold fi lms (Figure 1 a–b) with microscopic gaps (2–10 μ m) between two neighbor Au electrodes are used as substrates for fabri-cating sensor devices. The B-FTS technology (Figure 1 c) used a home-made setup using a simple burner with a one-direc-tional axial diffusion fl ame created by mixture of oxygen and propane gasses. The Zn precursor microparticles are inserted into a burner reservoir, and afterwards they are transported into the fl ame via a pure oxygen jet (Figure 1 c) and simultane-ously converted to metal oxide by the high-temperature fl ame in a normal environment. The converted nano-microstructures of zinc oxide are rapidly deposited on the patterned chip with gold electrodes (further details are described in the Experi-mental Section). Figure 1 d illustrates the schematic diagram of experimental setup of the C-FTS technique [ 37 ] for direct fab-rication of nano-microstructures on the chip (processing time ranges 0.5 to 4 hours). In C-FTS setup, patterned substrates are mounted on the top holes (symmetrically distributed) facing towards the fl ame generated by a crucible containing precursor material (mixture of Zn and sacrifi cial polymer-PVB). The precursor Zn microparticles are transported and simultane-ously converted into ZnO nano-microstructures by fl ame and are deposited on the chips. The schematic illustrations of ZnO nano-microstructures growth on pre-patterned chips by B-FTS and C-FTS techniques are shown in Figure 1 e and f, respec-tively. The B-FTS approach mainly results in formation of ZnO nanotetrapod bridging networks; however, C-FTS leads the growth of long interconnecting ZnO nano-microrods across the gaps (Figure S1, Supplementary Information). In terms of processing time, the B-FTS approach is believed to be the fastest synthesis technique ever reported, allowing deposition of the ZnO nanostructures directly on the chip within 3 to 5

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and C-FTS approaches, as presented in Figure 2 . Figure 2 a shows the SEM image of the ZnO deposited on the patterned chip using B-FTS approach. Optical microscopy image of the chip (B-FTS grown) is shown in inset of Figure 2 a, where a clear contrast is visible between ZnO deposited area and the pre-patterned Au electrodes on quartz substrate. As seen from Figure 2 a–b, nanotetrapod-shaped ZnO material was grown by B-FTS directly on the gap between two Au electrodes. The dimensions of the B-FTS ZnO nanotetrapod morphology are in nanoscale region (arm thickness ≈30–100 nm, and its lengths vary from 100–500 nm). The growth rate across the length of the arm of the tetrapod is estimated to be in the μ m s −1 regime, as the time of fl ight in the fl ame is less than 200 ms as reported in our previous work. [ 37 ] However, as in literature generally growth rates around 3.3 nm s −1 [ 38 ] to 33.3 nm s −1 [ 39 ] for ZnO structures are reported. It seems that at preliminary

seconds. Considering such a rapid crystal growth, we claim that B-FTS is one of the fastest and most effi cient technologies for ZnO nanotetrapods synthesis directly onto pre-patterned chips. Also, B-FTS technique allows ZnO nanotetrapods growth with high crystal quality and thickness of tetrapod arm below 50 nm. As already mentioned, the binding quality of contacts between nanostructures and electrodes have a very important role in the context of nanodevices performances. The binding qualities of contacts between ZnO nano-microstructures and Au electrodes for B-FTS and C-FTS fabricated specimens were investigated. Detailed SEM studies for both samples revealed good bindings between structures and electrodes as demonstrated by wetting of the ZnO at the bases of nano-microstructures (Figure S2, Supplementary Information).

ZnO nano-microstructures used in current UV photodetec-tion studies were grown on the Si and quartz chips using B-FTS

Figure 1. Basic concepts of photo-detection devices built from ZnO nano-microstructure network bridge by B-FTS or C-FTS approaches. a) Schematics of the ZnO nanostructures bridging two Au contacts and its arrangement for photo-detection measurement. b) Digital image of lithographically pre-patterned Au fi lm on quartz substrate and the magnifi ed region of Au electrodes. c,d) Schematic designs of the B-FTS and C-FTS approaches for growing ZnO nano-microstructures directly on the chip. e,f) Schematic illustrations of ZnO nano-microstructure grown by B-FTS and C-FTS techniques. B-FTS mainly results in the growth of ZnO nanotetrapod bridging networks and C-FTS gives long needles forming the conducting path through the gap between two Au contacts on the chip.

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ON forming nanojunctions directly during fabri-

cation process. At the same time, the nano-junctions between nanotetrapods provide a mechanical support to the networks bridging gap between the two Au electrodes and also establish a continuous path for electrons fl ow through it (Figure S3a,b, Supplementary Information). Figure 2 c presents overall I – V characteristics of the nanotetrapod bridging network device when UV light (≈15 mW cm −2 ) is switched on/off. From these data it can be concluded that the elec-trical conductance changes more than two orders of magnitude under UV illumination. Also, it can be concluded from Figure 2 c that two Schottky contacts are formed between ZnO nanotetrapods and metal contacts.

Figure 2 d,e shows SEM images of ZnO nano-microstructures grown by C-FTS approach on patterned chip (Si) for 4 hours. As per our experimental observations, synthesizing of such ZnO nanostruc-tures by C-FTS approach requires about 0.5 to 4 hours. Formation of large ZnO nanoneedles network (Figure 2 d) bridging the gap between two Au electrodes on the chip can be observed. Figure 2 e shows the high magnifi cation SEM image of the gap region shown in Figure 2 d. According to SEM image presented in Figure 2 e, across the gap region on the chip, long ZnO nano-needles from sides (on gold electrodes) inter-connect each other to bridge the gap, which is complete as a fi nal UV detector. Owing to their random alignments and higher aspect ratios, there is a high probability that ZnO nano-microneedles from both sides of the electrodes interpenetrate (partially or com-pletely) each other and establish a continuous path for current fl ow (Figure S3c,d, Supple-mentary Information). At the same time, some of the ZnO nano-microneedles can also form direct bridges between two Au elec-trodes. The growth mechanism is believed to be a combination of vapor solid (VS) and VLS process, but with the Au catalyst not coming at the top of the ZnO nanowires, as reported before. [ 21 ] The VS process takes place in the early stages of temperature ramping when the Zn vaporizes at around 550 °C, and it gets oxidized in presence of oxygen in air, just before starting formation of molten Au droplets at temperatures higher than

630 °C. [ 40 ] When the temperature of the furnace was reached ≈950 °C, the Au droplets are already formed and believed to serve as nucleation centers for growth of the ZnO nano-needles like conventional VLS mechanism but since amount of gold content is very large (gold coated electrode with thickness ≈35 nm), gold droplets do not reach the top of ZnO

stage within the fl ame, growth rate of tetrapods is much higher as compared to later stage when these nanotetrapods are depos-ited on the substrate but to some extent growth still remains continue, allowing interpenetration between tetrapod arms. [ 37 ] It was found that ZnO nanotetrapods form a bridging network across the gap on the chip by interconnecting their arms by

Figure 2. a–b) SEM images of ZnO nano-microstructures networks grown directly on the pat-terned chip (with Au electrodes) using B-FTS approach. The inset in a) shows the digital image of the chip and the bright (blue) contrast (in the centre) corresponds to location where these structures have been grown. b) High-magnifi cation scanning electron microscopy (SEM) image from the gap region in (a) showing growth of interconnected ZnO nanotetrapods, which form the bridge between the gaps. c) I – V characteristics from the bridging ZnO nanotetrapods net-work (shown in b) between the gap the in dark and under UV (365 nm) illumination. d–e) Low and high magnifi cation SEM images from 1D ZnO nano-microstructures bridging the gap grown by C-FTS approach respectively. Similarly to (a), the inset in (d) shows the digital image of the chip fabricated with ZnO structures by C-FTS approach. The high magnifi cation SEM image (e) demonstrates the growth of large ZnO needles which can easily bridge the gap. f) I–V characteristics from bridging ZnO needles network (e) grown by C-FTS approach in dark and under UV (365 nm) illumination. It shows that the on/off ratio of the bridging network (d) is enhanced by more than two orders of magnitude.

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d (002) value of ZnO depositions is 2.64 and 2.59 Å for investigated B-FTS and C-FTS samples, respectively. The lattice parameters c and a were obtained using standard rela-tions: [ 26,27,34 ] c = 5.29 ± 0.01 Å and a = 3.30 ± 0.01 Å for B-FTS samples and c = 5.18 ± 0.01 Å and a = 3.23 ± 0.01 Å for C-FTS samples. These lattice parameters are in good agree-ment with accepted values and prove the quality of obtained samples. The measured value of the c / a ratio for samples B-FTS and C-FTS was around 1.603 and 1.604, respec-tively, which is in agreement with value for bulk ZnO of 1.602. [ 34,42 ]

Raman measurements can provide detailed information about the material quality, the phase and purity, which are very helpful to understand transport properties and phonon interaction with the free carriers for determining device performance. [ 5,25,43–45 ] Therefore, Raman investigations of fabri-cated ZnO nano-micro-structures specimens by both approaches, B-FTS and C-FTS, were performed in the quasi-backscattering geom-etry. In our investigations, micro-Raman scattering was used to study the quality of the deposited ZnO nano-microstructures on Si-SiO 2 substrate bridging two Au electrodes.

The non-polarized Raman spectra are illustrated in Figure 3 c and 3 d for specimens grown by B-FTS and C-FTS approaches, respectively. The optical phonons at the Γ point of the Brillouin zone belong to equation (2) as below: [ 44–47 ]

�opt = 1A1 + 2B1 + 1E1 + 2E2 (2)

The E 2 (high) peak was one of the characteristic peaks of wurtzite ZnO attributed to the high frequency E 2 mode assigned to multiple-phonon processes. [ 46 ] Dominant peaks at 100 cm −1 and 437 cm −1 , which are commonly detected in the wurtzite structure of zinc oxide, are attributed to the low- and high- E 2 mode, respectively, of non-polar optical phonons [ 41,45,46 ] The peak corresponding to E 2 (high) mode has a line width of about 11 cm −1 which is comparable to values reported for good-quality ZnO bulk crystals. [ 24,26,41 ] The position of the E 2 (high) peak corresponds to the phonon of a bulk ZnO crystal, [ 46 ] indi-cating a strain-free state of the nanostructures. It was observed characteristic peaks related to ZnO, strong E 2 (high and low) modes at 437 cm −1 and 100 cm −1 , in addition to two weak E 1 (LO) at 584 cm −1 and second-order 2 E 2 mode at 331 cm −1 for B-FTS sample. An intense E 2 (high) mode peak in samples indi-cates that the wurtzite crystalline quality of the ZnO is high. All of the scattering peaks are assigned to ZnO. [ 41,42 ] Thus, the Raman spectroscopy study confi rms a high crystalline quality of the nanostructures observed from XRD studies.

As already presented in Figure 2 b, high magnifi cation SEM investigations prove that bridging network across the gap (B-FTS) is entirely made from interconnected nanotet-rapods (Figure S3a,b, Supplementary Information). Figure 4 shows the corresponding photo-detection measurements of ZnO nanotetrapod bridging gaps between two Au electrodes

nano-microneedles. Zinc oxide nanoneedles with lengths in the range of 50 to 200 μ m and diameters in the range of 50 to 200 nm which form interconnecting bridges across the gaps were synthesized by C-FTS approach (Figure 2 e; Figure S3c,d, Sup-plementary Information). The I–V characteristics under dark and UV (365 nm) irradiance shows more than two orders of magnitude in the on/off ratio (Figure 2 f) and ohmic contacts behavior in C-FTS samples.

The crystalline nature and growth orientation of these ZnO nano-microstructures (grown by B-FTS and C-FTS approaches) on SiO 2 /Si substrates with Au electrodes on chip was confi rmed by X-ray diffraction (XRD) studies. XRD spectra ( Figure 3 a,b) show that ZnO nano-microstructures do not exhibit a dominant Bragg refl ection in all depositions of the samples by B-FTS and C-FTS techniques. As can be seen from Figure 3 a,b, the main XRD peaks for B-FTS and C-FTS samples are (100) and (002), and refl ections (101) are lower. The Bragg refl ections that cor-respond to ZnO nanostructures, Si substrate and Au contacts are marked in Figure 3 a,b, respectively. The high intensity and narrow full width at half maximum (FWHM) XRD peaks of the samples obviously reveals that the ZnO are of high crystal quality after synthesis by B-FTS and C-FTS processes. In the hexagonal structure of ZnO, the plane spacing is related to the lattice constant a and the Miller indices by equation (1): [ 26,27,41 ]

1

d2(hkl )

= 4

3

(h2 + hk + k2

a2

)+ l 2

c2

(1)

where a and c are the lattice constants, d is the spacing between planes of given Miller indices h , k , and l . The lattice parameter d (002) in the non-stressed ZnO bulk is about 2.602 Å, and the

Figure 3. a–b) XRD and c–d) micro-Raman studies of ZnO nano-microstructure network bridges fabricated on SiO 2 /Si substrates with Au electrodes by B-FTS and C-FTS approaches, respectively.

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ON curves of electrical current in dark and under

UV illumination of the C-FTS device at con-stant 0.3 V bias voltages. A current on/off ratio of ≈312 at 0.3 V bias voltage was meas-ured for C-FTS device. A closer investigation of a single pulse photocurrent shows that the rise and decay photocurrent follow exponen-tial function. The rising and decaying pho-tocurrent (Figure 4 b,d) with corresponding time measurements are determined through bi-exponential fi t respectively. The two bi-exponential function equations (3,4) used for rising and decaying photocurrent, [ 44,45 ] respec-tively are:

I(t) = I0 + A1(1 − e− tJr 1 ) + A2(1 − e− t

Jr 2 )

(3)

I(t) = I0 + A3e− tJd1 + A4e− t

Jd2 (4)

where I 0 is dark current, A 1 , A 2 , A 3 and A 4 are positive constants. τ r1 , τ r2 and τ d1 , τ d2 are time constants for rising and decaying photocurrent, respectively. On the basis of the curve fi ttings, the rise time constants for B-FTS specimens were τ r1 = τ r2 = 68 ms, and decay time was τ d1 = 32.18 ms. How-ever, for the decay part of the curve, it can be seen that second time constant is large, τ d2 = 200.48 ms. Similar fi ttings were performed for C-FTS samples and resulted in rise time τ r1 = τ r2 = 27 s and decay time constants

τ d1 = 7 s and τ d2 = 40 s. According to our experimental results presented above, it can be concluded that B-FTS samples shows an excellent response/recovery time, and the fastest rise time constants of about 68 ms and fi rst decay time of 32 ms. For comparison, C-FTS synthesized samples shows much slower total response/recovery times of about 27 s and 7 s, respectively.

Table 1 summarizes earlier reports on ZnO-nanostructures based UV photodetectors [ 11,35,45,48,49 ] and gives a comparison to our B-FTS and C-FTS devices. As compared to most of the pre-vious reports, our B-FTS UV photodetectors show the best per-formances in terms of fastest rise/decay times at higher signal ratio ever reported. Law et al [ 45 ] reported the fastest rise/decay times for their UV photodetectors, but they used multiple pat-terning steps followed by evaporation for ZnO nanowire device

on pre-patterned quartz substrate (Figure 1 a,b, SEM image in Figure 2 a). Figure 4 a presents the reversible switching curves of current through the device structure when the UV light was switched on/off every 50 seconds, at a constant bias voltage of 3.6 V across it. The reproducibility of the photocurrent variations is demonstrated by almost identical rises and decays curves in four cycles. The on/off ratio is ≈254.5 at 3.6 V biasing and it increases to ≈3277 at 3.0 V or to ≈4500 at 2.4 V biasing of the device structure. The ZnO nanotetrapods network-based device was found to exhibit a fast response time to UV light irradia-tion and very fast recovery time. Our experimental results clearly prove the promising potentials of the B-FTS technology for rapid fabrication of highly effi cient UV photodetectors devices based on networked ZnO nanotetrapods. Figure 4 c shows switching

Table 1. ZnO nanostructure-based photodetectors.

Type of nanostructure Duration of growth & method UV light Bias voltage I UV / I dark Rise time Decay time Ref.

Single nanowire 30 min, CVD 365 nm − 1.25 2 min 3 min [ 35 ]

Single nanowire 2–10 min, VLS 365 nm 0–5 V 2.5 × 10 5 ≈1 s ≈1 s [ 11 ]

Nanorod network 3–5 h, HT 365 nm 4 V 1.8 2 s − [ 49 ]

Nanowire array 3 h, TE 370 nm 5 V 1.4 0.1 ms 0.4 ms [ 45 ]

Spatial tetrapod network 38 min, TE 365 nm 1 V 10 6 0.4 s 0.3 s [ 48 ]

Nanoneedle network 4 h, C-FTS 365 nm 0.3 V 312 22 s 7–12 s This work

Nanotetrapod network <5 s, B-FTS 365 nm 2.4 V 4.5 × 10 3 ≈67 ms ≈30 ms This work

Figure 4. a) Reversible switching of electrical current at 3.6 V biasing voltage with 50 s peri-odic illumination of UV light for B-FTS specimen. b) The rise and decay of the current in time and time constants measured through bi-exponential fi t for a single pulse in (a). c) Reversible switching of electrical current for C-FTS detector at 0.3 V biasing voltage in the dark state and under UV light irradiance. d) The rise and decay of the current in time and time constants measured through bi-exponential fi t for a single pulse in (c).

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Information). Other types of nanojunctions can be formed directly inside of the nano-tetrapod at the connections of four different nano-arms. As presented above, it is believed that UV detection performances strongly depend on the type of such nanojunctions formed, boundary layers that are formed and energy barriers between nanotetrapods bridging two electrical electrodes. Electrical properties of such boundary of the nano-junctions in semiconducting ZnO could be mainly responsible for enhancing the electrical activity of the interfaces between nanoarms in a bridging network. It can be explained by fact that nanojunction boundary becomes electrically active by trapping an excess charge of majority carriers (e − ) at the interface. Total charge neutrality is formed by an electrostatic potential barrier between neighboring depletion layers adjacent to the nanojunction (Figure 5 b,d). Considering completely ionized shallow donors of density N 0 for the depletion layers in semiconducting zinc oxide, the maximum band bending Φ B ( V = 0) at the nanojunction boundary is calculated from Poisson’s equation [ 50 ] :

�B (V = 0) =Q2

i

8egg0 N0 (5)

where e is the electron charge, ε is the static dielectric constant, and ε 0 is the permittivity of a vacuum.

Thus, the width of the depletion region is

x0 =Qi

2N0≈

√[g�B

N0

]

(6)

Equations ( 5) and ( 6 ) permit understanding of effects at the grain boundaries in the networked tetrapods/wires. Thus, bar-rier height Φ B ( V = 0) ≈ 1 V is reduced under UV irradiation of nanotetrapod junctions and leads to a faster fi lling of the inter-face states for applied bias ( V > 0). The bottom schematics in Figure 5 b present structural models of the conduction channel of nanojunctions between nanowires in air for touching nanowires. Interpenetrating nanowires are also represented by the bottom schematics in Figure 5 d. Formation of the deple-tion layer and its characteristics were reported in details in pre-vious works. [ 5,35,42 ] The corresponding schematic energy band diagram (top) in the radial direction of a single ZnO nanowire, indicating the depletion region at the surface, the surface band bending and the quasi-neutral core region of radius r in the central part of the nanostructure are also shown in Figure 5 b. It can be seen that the conduction channels are still not con-nected or that they are separated by the depletion layer region, although the size of conduction channel of nanowires increased under UV irradiance because of releasing of free electrons from oxygen ions (initially adsorbed on the ZnO surface) (Figure 5 b). A high potential barrier to the surface states exists because of

fabrication. In comparison to earlier reports, our B-FTS photo-detectors are superior in terms of higher signal ratio ( I uv / I dark ) (3288 times in our work and 1.4 times in the previous report, [ 45 ] rapid synthesis time and simplicity of fabrication technique with direct integration of ZnO nanostructures on a chip. We would also like to emphasize that B-FTS specimens based on interconnected-interpenetrating crystalline ZnO nanostructures networks with fast recovery time constants ( ∼ 32 ms) could be limited in speed studies owing to the resolution limit of our measuring setup.

The possible photo-detection mechanism for such high current ratio ( I uv /I dark ) and very fast response/recovery times of our devices can be explained by using schematic drawings like presented in Figure 5 . In Figure 2 b it was observed that a large number of nanostructures with various morphologies being interconnected in a random fashion. These indicate on the possible presence of several types of nanojunctions (Figure 5 ) between interconnecting-interpenetrating neighbor tetrapods nanoarms or nanowires forming network bridge in the gap. In particular, it can be distinguished between a just touching nano-arms (Figure 5 a), or partly interpenetrated nano-arms (Figure 5 c) of nanotetrapods synthesized through B-FTS (Figure S3a,b, Supplementary Information). Similarly, explanations can be extended to nano-microneedles synthe-sized through C-FTS approach (Figure S3c,d, Supplementary

Figure 5. Schematic representation of: a) touching tetrapod arms, and b) its corresponding (top) energy band diagram in the radial direction and (bottom) cross-section structural models of the conduction mechanism of networked ZnO nanowires forming potentials barriers at the boundary. A high potential barrier to the surface states because of the depletion electrons by desorbed oxygen molecules on the surface of touching nanowires. Under UV illumination, the height of the potential barrier decreases but still signifi cantly large (top). c) Shows the case of network bridge in which tetrapod arms are interpenetrating each other and its corresponding d) (top) energy band diagram in the radial direction and (bottom) cross-section structural models The right schematics in d) presents reduced potential barrier at the boundary of two interpenetrating nanowires in cross-section that are irradiated with UV light (top) signifi cantly increased channel and thus connected channel (bottom).

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ON electrode was presented in Figure 1 a. The B-FTS experiment utilizes

one-directional axial diffusion fl ame with oxygen jet feeding the fl ame besides enhancing the combustion of Zn microparticles (size 1–5 μ m, GoodFellow GmbH, Bad Nauheim, Germany). The fl ame converts Zn microparticles into zinc oxide nanotetrapods and carries it to the micro-patterned substrate with Au electrodes. In case of the B-FTS approach, the substrate is coated with nano-microstructures within 3–5 s.

Schematics of experimental setup for C-FTS approach was presented in Figure 1 b. The recipe for C-FTS involves a precursor material with 1 g of Zn micropowder (1–5 μ m size) in the bottom of ceramic crucible and 3 g of (PVB+Zn) of ratio 2:1 on the top. The ceramic crucible fi lled with PVB + Zn is heated with average temperature rate of 58 °C min −1 in a muffl e type furnace from room temperature to 950 °C. After reaching 950 °C, the furnace is maintained at this temperature for different durations (0.5 to 4 hour) and the nanostructure growth process takes place. The best and effi cient arrangement for the catalytically assisted nanostructures growth is experimentally found in an arrangement where 5 holes, numbered as 2 are left open and four holes numbered as 1 in Figure 1 b are covered with the catcher to collect the nanostructures. The samples were placed with Au contact side down so that the microstructured Au electrodes face the metal-oxide vapor during growth process. The crucible has the following geometric dimensions: height 38 mm, top diameter 30 mm and bottom diameter 18 mm (Eydam, LLG GmbH, Kiel). The ceramic tube has 50 mm diameter and 60 mm height (Friatec, Alsint, Germany).

Device Characterization : The overall microstructural evolution was investigated using an optical microscope. The nanostructures morphologies were investigated and characterized using scanning electron microscopy (SEM) instrument Dualbeam Helios Nanolab (FEI) (20 kV, 20 μ A) and with a Zeiss Ultra Plus SEM instrument. The compositional analysis of ZnO deposition was carried out using EDX, in combination with SEM. X-ray diffraction (XRD) studies were performed by using 3000 PTS Seifert XRD unit (with 40 kV and 40 mA, Cu K α radiation with λ = 1.541 Å). The average crystal lattice parameters were determined by the Scherrer method from the diffraction peaks. [ 41 ] Raman scattering measurements were performed at room temperature with a WITec LabRam Alpha 300 system in a backscattering confi guration. A 532.2 nm line of Nd-YAG laser was used for off-resonance excitation with less than 4 mW power at the sample. The instrument was calibrated to the same accuracy using silicon and a naphthalene standard. Keithley 2400 source meter (Keithley instruments, Germany) has been used for the electrical and photocurrent measurements. The detection of UV light is performed through using H135 UV source, Labino Compact UV lamps (Labino AB, Sweden). The Spotlight at ≈3.5° distribution angle (beam) provides irradiance ∼ 45 mW cm −2 at 38 cm height from the sample to the lamp. The height at which our UV detection measurement was carried out was 8 cm. Practical intensity measurements at a height of 8 cm through using a photodiode show a range of value 15–20 mW cm −2 .

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements This research was sponsored by German Research Foundation (DFG) under the schemes SFB 855-A5 and partially by SFB 677-C10. Y.K.M. and O.L. acknowledge the Alexander von Humboldt Foundation for the fi nancial support through research fellowships at the Institute for Materials Science, University of Kiel, Germany.

Received: August 30, 2013 Revised: October 10, 2013

Published online: November 18, 2013

the depleting electrons by oxygen on the surface of interpen-etrating nanowires. [ 5,12,14 ] However, with irradiance of the interpenetrating ZnO nanowires forming nanojunctions using UV light (365 nm), the potentials barriers are signifi cantly reduced and the channel width becomes interconnected as schematically shown in Figure 5 d. Unlike the case of the inter-penetrating ZnO nanostructures, it can be concluded that the UV irradiation does not infl uence signifi cantly energy barrier between touching nanowires and electrons do not have a free conduction path. By contrast, contact that is too intimate with a perfect interpenetration would also not change much in con-ductivity under UV irradiation as no barrier has to be overcome. So the ideal sensitive contact would be something in between a slight touching and a complete interpenetration. Thus, it can be concluded that types of nanojunctions formed between nano-microstructures in the network bridge have signifi cant roles in photodetection mechanism of such photodetectors.

In conclusion, we present two new methods (B-FTS and C-FTS) for rapid growth of high-quality ZnO nano-microstruc-tures directly integrated into the chip in device form. The dem-onstrated experimental details provide that both approaches allow us in situ integration of nano-microstructures between the electrodes on chips during the growth process itself. The B-FTS technology can be characterized by a rapid synthesis process (3–5 seconds) for fabrication of highly crystalline inter-connected ZnO nanotetrapod-network bridging the two pre-pat-terned Au electrodes. B-FTS fabricated photo-detector devices exhibit a high current ratio I UV / I dark ≈4.5 × 10 3 at 2.4 V with a fast response/recovery times (≈68 ms, ≈32 ms). The UV pho-todetection mechanism is explained in terms of modifi cations in energy band diagrams under UV illuminations of different ZnO nanojunctions. In such cases, energy barriers affect the current fl ow and thus detection ( I UV /I dark ) of UV light. Pro-posed technological approaches can be easily scaled-up and therefore exhibit large interest for further studies in UV detec-tors and for various sensor applications.

Experimental Section Device Fabrication : Device preparation begins with preparation of

Au electrodes. To prepare Au electrodes with a gap size of 2–10 μ m between them conventional photolithography was used on Si-SiO 2 or quartz substrates. The initial stage of the pre-patterned chip preparation involves micro-structuring of a photoresist AZ MiR 701 (Micro Chemicals GmbH, Ulm Germany). Using magnetron sputtering was deposited a 5 nm Cr fi lm for better adhesion of Au to the substrate and subsequently a 35 nm thick Au fi lm on top for electrical contacts. Similar pre-patterned electrodes were made on quartz substrate (thickness 2 ± 0.3 mm, from GVB GmbH– Solutions in Glass, Germany) and <100> Si substrate (≈406 μ m thick, P/Boron) coated with a 300 nm thermally oxidized SiO 2 (Active Business Company GmbH, Germany). After photoresist mask-lift off using acetone in ultrasonic bath, the substrates were transferred for synthesis of nanowires/nanotetrapods by B-FTS or C-FTS approaches. The synthesis of nano-microstructures in this work was performed in air environment and is fundamentally based on fl ame induced conversion and simultaneous transport of ZnO to the catcher substrate (chip with pre-patterned electrodes). As reported above, both B-FTS and C-FTS approaches allow growing of ZnO nanowire/nanotetrapod bridges over the gaps between two Au electrodes.

The schematics experimental setup for B-FTS process used in the ZnO deposition through the fl ame onto micro-patterned Au

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