896 ieee journal of selected topics in quantum electronics...

896 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 17, NO. 4, JULY/AUGUST 2011

Optical Applications of ZnO NanowiresApurba Dev, Abdelhamid Elshaer, and Tobias Voss

(Invited Paper)

Abstract—This paper discusses different aspects of optical appli-cations of ZnO nanowires NWs. After a description of the relevantsynthesis and fabrication techniques, light-emitting diodes basedon ZnO NW and NW arrays are introduced and different experi-mental realizations from the literature are discussed. The workingprinciple of ZnO UV photodetectors is presented, and improve-ments and limitations of ZnO- NW-based dye-sensitized solar cellsare discussed. Different aspects of ZnO-NW waveguides and theirpotential application for biological sensing are described. Finally,the current status of ZnO-NW-based UV lasers is presented.

Index Terms—Nanowire (NW), optics, semiconductor, zincoxide.

I. INTRODUCTION

THE growing demand of ultralight, smart, and multifunc-tional devices together with faster information-processing

ability has led to the discovery of many new optical materialsand the miniaturization of optoelectronic devices that also fea-ture better performance, low cost, and low power consumption.The research effort for the past two decades has led to a richcollection of nanostructures that is suitable for many optical ap-plications. Among various nanomaterials, semiconductor NWsoffer opportunities to assemble nanoscale devices for the pho-tonics and electronics platforms. As a result, they have drawnintense research interest. Obviously, the realization of nanoscaledevices not only requires suitable materials for specific appli-cations, but also the ability to control key parameters of thenanostructures including their size, shape, spatial distribution,and doping. ZnO NWs represent one of the nanomaterial sys-tems, where these key parameters have been best studied andcontrolled to date.

The interest in ZnO can be traced back almost half a cen-tury [1]. Yet, the lack of stably p-doped ZnO material thatcan be fabricated in a reproducible way has thus far hinderedthe commercial fabrication of ZnO-based optoelectronic de-vices [2]–[4]. Compared to its direct competitor, GaN, the non-toxic ZnO offers the advantage of wide bandgap (3.37 eV) andhigher exciton binding energy (∼60 meV [1]) that ensures effi-cient exciton emission above room temperature. In addition, thelack of a center of symmetry in wurtzite crystals, combined withstrong electromechanical coupling, results in large piezoelectric

Manuscript received July 13, 2010; revised August 11, 2010; acceptedAugust 24, 2010. Date of publication December 6, 2010; date of current versionAugust 5, 2011.

The authors with the Institute of Solid State Physics, University of Bremen,D-28359 Bremen, Germany (e-mail: [email protected], [email protected], [email protected]).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/JSTQE.2010.2082506

and pyroelectric coefficients and the use of ZnO in mechanicalactuators and piezoelectric sensors [5]. One of the potential ap-plications for ZnO is to replace indium-tin-oxide (ITO) as themost commonly used transparent conductive oxide because ZnOstays transparent, even when doped above the level of degen-eracy. ZnO is relatively cheap and available in large quantities,two facts that clearly distinguish it from ITO.

The renewed interest in ZnO has been mainly fueled by thedevelopment of synthesis methods for 1-D nanostructures likeNWs. Nowadays, ZnO NWs can be easily synthesized with pre-cise control over their dimensions. Since the discovery, manyfascinating applications have been developed [6], [7] and theresearch on the ZnO NW system has become a leading fieldin nanoscience and nanotechnology. Applications of ZnO NWsinclude electronic and sensing devices like chemical and biolog-ical sensors exploiting the large surface-to-volume ratio of theNWs; electromechanical sensors, transducers, and piezoelec-tric generators utilizing their piezoelectric and semiconduct-ing properties; transparent and flexible electronics with NWtransistors, diodes, and field emitters. Concerning their opti-cal properties, ZnO NWs are well-suited for a large numberof applications like LEDs [8]–[10], UV photo detectors, andoptical switches [11]–[14], dye-sensitized solar cells (DSSCs)[15]–[18], optical waveguides [19]–[22], nonlinear frequencyconverters [23]–[28], and UV lasers [29]–[36]. NW-like struc-tures are also the ideal system for studying the transport pro-cesses in 1-D confined objects, which are of benefit not only forunderstanding the fundamental phenomena in low-dimensionalsystems, but also for developing a new generation of nanode-vices [37], [38].

II. SYNTHESIS

A. Synthesis of ZnO NWs

The intense interest in ZnO NWs for optical applicationshas prompted the development of a wide variety of differentfabrication techniques with precise control over size, shape,orientation, and distribution [39]–[65]. Scanning electron mi-croscope images of ZnO nanowires grown by different tech-niques are shown in Fig. 1. ZnO NWs are nowadays read-ily available in large quantities and with excellent crystallinequalities. Among various fabrication techniques, the vapor–liquid–solid (VLS) growth has achieved the most success inproducing high-quality ZnO NWs. This technique promotesoriented growth by introducing a catalyst in liquid form thatcan rapidly adsorb a vapor to supersaturation levels. The VLSmechanism, proposed by Wagner and Ellis [39], has been laterdeveloped by many other research groups [40]–[44]. The di-ameter of VLS NWs can be controlled by tuning the size of

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DEV et al.: OPTICAL APPLICATIONS OF ZnO NANOWIRES 897

the metal droplets. Their length can reach up to hundreds ofmicrons. A uniform size distribution of the NWs can be obtainedby using monodispersed metal nanoparticles (NPs) [45]. Preciseorientation control during NW growth can be achieved by apply-ing conventional epitaxial crystal growth techniques to the VLSprocess. Although the VLS process has been very successfulin producing high-quality ZnO NWs, the use of metal catalystsinevitably introduces the problem of a potential contamination.This has been a key concern when considering NW technolo-gies for the optoelectronics industry. To resolve this problem,catalyst-free vapor-phase growth has been developed [46]. Thisapproach is similar to that of the VLS growth technique ex-cept that no catalyst is needed. However, all the vapor-phasetechniques usually require high growth temperature (450 ◦C–1100 ◦C) and often face other limitation in terms of sample uni-formity, choice of substrate, and low yield. To avoid these limi-tations, solution-based approaches have been developed, whichhave the advantages of low growth temperatures (<300 ◦C), andthe potential for scaling up and producing high-density arrays(NW number density >1010 cm−2) [47].

The wet-chemical synthesis of ZnO NWs has been exten-sively investigated in last couple of years [47]–[67]. In solution-based methods, a zinc salt is dissolved in water, giving rise tovarious hydroxyl species [49]. However, the stability of thesecomplexes depends on the pH value and the temperature of thesolution [50]. At elevated temperatures, solid ZnO nucleationcenters are formed by the dehydration of the hydroxyl species,and the ZnO crystals continue to grow by further condensation ofhydroxyl groups [50]. In general, NW growth is possible underslightly acidic to basic conditions (5 < pH < 12) at temperaturesfrom 50 ◦C to 200 ◦C. In basic environments (pH > 9), wiresare formed, even in the absence of additives [51]–[53]. How-ever, for growth in acidic environments (pH < 9), an additive,such as hexamethylenetetramine (HMTA) or dimethylamineb-orane, must be used to promote 1-D growth of ZnO [50]. Ingeneral, rods (aspect ratio < 10) and wires (aspect ratio > 10)form in the presence of amines, such as HMTA [47], [53]–[56],ethylenediamine [57]–[59], triethanolamine [58], and diethylen-etriamine [47], [60]. The most successful and widely used chem-ical method for ZnO NW growth is the hydrolysis of zinc ni-trate in water in the presence of HMTA [53]–[61]. In addition,growth from a seeded substrate has also been a subject of in-tense research interest, as highly oriented NW arrays can begrown following the crystallographic orientation of the seedlayer [62]. Several studies [55]–[63] have shown that smallerseeds yield thinner wires. However, the HMTA-mediated syn-thesis lacks the ability to produce wires with high aspect ratios(>50), as the growth along the length (c-axis) is usually followedby lateral growth. The introduction of an additional moleculeis required that can restrict the radial growth. Both amines anddiblock copolymers [64] have been shown to do this. By theaddition of low molecular-weight poly(ethylenimine) to the so-lution, it has been demonstrated that long NWs are formed (upto 40 μm) with aspect ratios up to 330 [65] by adjusting thereactant concentrations, the temperature, and the growth time.Another way to achieve a high aspect ratio is to use appropriatesurfactants to restrict lateral growth [62]. It is known that above

a certain concentration, known as the critical micelle concen-tration, the surfactant can form micelles of a special shape (e.g.,rod, spheres, etc.) [52]–[66]. The micelles are usually stable atlow temperatures [52]–[66] (<100 ◦C). The rod-like micellesformed in such cases can entrap the Zn2+ and OH− ions fromthe solution producing Zn(OH)2 and finally decompose to ZnOat elevated temperature. The elevated temperature is believedto increase the activity of the surfactant, causing the micellesto collide with each other, thus, increasing the length of thenanorods [62]. Very recently, Xu et al. demonstrated [67] an ap-proach for growing uniform and horizontally aligned ZnO NWarrays by hydrothermal decomposition at a temperature lowerthan 100 ◦C. This technique is very suitable to manipulate NWs,even at lower temperature, which could lead to the realizationof flexible electronics.

III. OPTICAL APPLICATIONS

A. Nanowire (NW) LEDs

To meet the challenges of the growing demand of energy,the importance of research in solid-state lighting has signifi-cantly increased, as it is expected to reduce the global electric-ity consumption of lighting by 50% [68]. However, despite itsimmense potential as an efficient alternative to the conventionallight sources, the LED technology suffers serious challenge interms of poor extraction efficiency and intensity dependenceon the junction area. A large part of the current research onLED materials is thus heavily focused on dense NW arrays,where both of those challenges can be easily addressed. Thewide-bandgap ZnO NWs offer many advantages as a suitableLED material. The large exciton binding energy, large index ofrefraction (n > 2) ensuring strong photonic confinement, lowcost, and easy fabrication processes make it a preferable can-didate. As a result, ZnO-NW based LEDs have been widelyinvestigated [8]–[10], [69]–[75] despite the lack of reliable p-doping. Although there are few reports on ZnO-NW homo-junction LEDs [69], [70], most of the investigations are basedon hybrid structures using a polymer as the p-conducting mate-rial [8]–[10], [71]–[75]. In principle, such hybrid structures offerseveral unique advantages: the polymers, usually applied in aliquid form, can penetrate the dense arrays of NWs and, thus, canform the junction on almost the entire NW surface. The struc-ture also offers wavelength tunability that can be achieved byusing different polymers. In addition, this technique is suitablefor scaling up at low cost.

A schematic of a ZnO NW/polymer heterojunction diode isshown in Fig. 2. The device consists of a dense array of ZnONWs grown on an ITO substrate by a chemical technique [74].Prior to the NW growth, a thick layer (∼500 nm) of ZnO NPswas deposited from solution. A hole-conducting polymer, poly(3, 4 ethylenedioxythiophene):poly (styrenesulfonate) widelyknown as PEDOT:PSS, was then spin-coated onto the NWs.Finally, the top electrode was made by depositing a thin layerof gold.

The I–V characteristics of the NW LED in both forwardand reverse directions are shown in Fig. 3 (inset: currenton a logarithmic scale). The ZnO/PEDOT:PSS heterojunction


Fig. 1. Scanning electron microscope (SEM) images of ZnO NWs grown by(a)–(c) various wet-chemical techniques and (d) vapor-phase method. Horizontaland vertical alignments of NWs are shown in (e) and (f), respectively. Imagesource: Image (a), (b), (d), (e), and (f) are reproduced with permission from[62], [65], [44], [67], and [93], respectively. The scale and the index have beenprovided by the authors.

Fig. 2. Design scheme of the LED structure consisting of ZnO NWs grownon an ITO-coated glass substrate. The top contact consists of a p-type polymer(PEDOT:PSS) and an evaporated Au layer.

Fig. 3. I–V characteristics of a ZnO/PEDOT:PSS heterojunction. Inset: currenton a logarithmic scale.

clearly demonstrates good rectifying behavior with a rectifica-tion factor of about 25 at ±5 V. Fig. 4 shows an electrolumines-cence (EL) and a photoluminescence (PL) spectrum of the ZnONW LED. The EL spectrum was recorded at room temperatureunder 6 V. In comparison to the PL spectrum of the NW array,the EL from the device shows a broad luminescence (380–820

Fig. 4. PL and EL spectra of a ZnO/PEDOT:PSS heterojunction LED. Forthe PL measurement, the sample was excited at room temperature with a HeCdlaser at 325 nm.

nm) almost covering the entire visible spectrum and giving riseto the apparent white color of the emission. However, the deviceshows negligible UV emission in EL. Its operation was stablefor more than 5 h under ambient conditions. Though the mech-anism behind the broad EL spectrum is not well understood, itis believed that the ZnO/polymer interface produces a strongerdefect luminescence due to the surface modification [76]. Thismay also lead to the quenching of the near-band-edge emissionin the composite device: whereas the PL originates from op-tically and homogeneously excited NWs, the EL is created atthe surface of the NWs. Furthermore, the PEDOT layer mightalso contribute to the EL emission. However, tests with differentsamples in which the PEDOT layer was directly deposited ontothe ITO substrates showed that this direct contribution from thePEDOT itself should be negligible for EL generation.

Strong UV emission from a similar device was observed byKonenkamp et al. [73] when the NWs were annealed prior tothe device fabrication. This has been assigned to the efficientinjection and transport processes in the annealed NWs. It hasalso been possible to fabricate the entire device on a flexiblesubstrate [75]. In many cases [73], [74], a thin insulating layer(polystyrene, PMMA) was applied on top of the NWs to preventshort-circuits. This layer reduces the overall junction area.

However, investigation on a single NW p–n junction (in thiscase with an n-ZnO NW on a p-Si substrate) [10] showed thatthe thin insulating layer plays a very important role in obtain-ing a proper band alignment across the heterojunction. Tunnelinjection of holes into the ZnO NWs was only achieved withthe help of the insulating layer, and the thus produced ZnO-NWLEDs even showed very strong UV emission at room tempera-ture. In addition, the insulating layer may also help in reducingthe surface damage caused by the acidic PEDOT/PSS solutionthat will otherwise etch the surface of the ZnO-NW array. Theresults demonstrate that the ZnO NW/polymer composite p–njunctions are a promising platform for the fabrication of efficientUV or white LEDs.


Fig. 5. (a) I–V characteristics of a single-NW photodetector as a functionof light intensity. The curves were measured at the following intensities: 4 ×10−2 W/cm2 (black), 4 × 10−3 W/cm2 (red), 4 × 10−4 W/cm2 (green), 1.3 ×10−4 W/cm2 (blue), 4 × 10−5 W/cm2 (cyan), 1.3 × 10−5 W/cm2 (magenta),6.3 × 10−6 W/cm2 (yellow), and in dark (brown). Inset is the SEM image ofa typical ZnO NW; the spacing between the interdigitated electrodes is 2 μm.(b) I–V curves presented in (a) are replotted on a natural logarithmic scale.Image source: Image reproduced with permission from [14].

B. UV Photodiodes and Optical Switches

The conductivity of ZnO NWs is extremely sensitive to theexposure of UV light. In the dark, VLS-grown ZnO NWs can behighly insulating with a resistivity above 3.5 MΩ·cm, whereasupon exposure to UV light, the resistivity of the NWs decreasesby typically four to six orders of magnitude [11]. Such a strongphoto-induced conductivity change has been subject of exten-sive investigations [11]–[14] for applications like UV photode-tectors and optical switches.

A typical photocurrent measurement performed on a single-NW device under ambient environment is shown in Fig. 5 [14].The I–V characteristics of the device recorded under dark andUV illumination with different light intensities [see Fig. 5(a) and(b)] show an almost two to five orders of magnitude increase inthe current, as the light intensity is raised from 6.3 μW/cm2 to40 mW/cm2 .

The photoconduction mechanism of ZnO NWs is mainlygoverned by the chemisorption of oxygen [11], [12], [14], asschematically shown in Fig. 6. In the dark [see Fig. 6(b)], oxy-gen molecules are adsorbed on the oxide surface and capturethe free electrons present in the n-type oxide semiconductor[O2(g) + e− → O−

2 (ad)]. A low-conductivity depletion layer isformed near the surface. Upon illumination with a photon energylarger than Eg [see Fig. 6(c)], electron-hole pairs are photogen-

Fig. 6. Photoconduction in NW. (a) Schematic of a NW photoconductor. Uponillumination with photon energy above Eg , electron–hole pairs are generatedand holes are trapped at the surface. Under an applied electric field, the unpairedelectrons are collected at the anode, which leads to the increase in conductivity.(b) and (c) Trapping and photoconduction mechanism in ZnO NWs: the topdrawing in (b) shows the schematics of the energy band diagram of a NW in dark,indicating band-bending and surface-trap states. The bottom drawing showsoxygen molecules adsorbed at the NW surface that capture the free electronpresent in the n-type semiconductor forming a low-conductivity depletion layernear the surface. (c) Under UV illumination, photogenerated holes migrate tothe surface and are trapped, leaving behind unpaired electrons in the NW thatcontribute to the photocurrent. Image source: Image reproduced with permissionfrom [14].

erated [hν → e− + h+ ] and holes migrate to the surface alongthe potential slope produced by the band bending.

They discharge the negatively charged adsorbed oxygen ions[h+ + O−

2 (ad) → O2(g)]. Consequently, oxygen is photodes-orbed from the surface and the unpaired electrons are eithercollected at the anode or recombine with holes, which aregenerated when oxygen molecules are readsorbed and ion-ized at the surface. It is well known that photoconductors withblocking contacts, i.e., with a Schottky barrier at the metal-electrode/semiconductor interface, can exhibit hole-trapping atthe reverse-biased junction that shrinks the depletion regionand allows tunneling of additional electrons into the photocon-ductor; if electrons pass multiple times, this mechanism yieldsphotoconductive gain greater than unity [77]–[79]. In case ofZnO NWs, the highest photoconductive gain reported to dateis G = 2 × 108 [14]. The photoresponse time in such a deviceis slow with the fastest response time reported so far being∼0.4 ms [12]. However, the response time strongly depends onthe surrounding environment, e.g., the photocarrier lifetime isconsiderably enhanced in oxygen-deficient environments. Theseobservations are consistent with the proposed photoconductiv-ity mechanism involving oxygen adsorption and desorption atthe NW surface.

C. ZnO-NW-Based DSSCs

Although solar cells could offer the possibility of abundantand clean energy to satisfy the global demand, the fabricationof efficient and inexpensive solar cells still remains a challenge.Despite many years of research, silicon-based solar cells con-tinue to be expensive in addition to their limited conversionefficiency. To solve these problems, DSSCs have been pro-posed which are promising for inexpensive, large-scale solarenergy conversion. The conventional DSSCs consist of a film of


Fig. 7. Schematics of a NW-dye sensitized solar cell. Image source: Imagereproduced with permission from [15].

semiconductor NPs (usually TiO2 or ZnO) coated with a light-absorbing dye and embedded in an electrolyte that provideshole conduction. Recently it has been demonstrated that theefficiency of the DSSCs can be further enhanced by replac-ing the NP film by a dense array of ZnO NWs [15]–[18]. Inthe NP-based DSSCs, the electron transport process relies ontrap-limited diffusion, which is a slow mechanism with a highelectron annihilation rate strongly limiting the device efficiency.On the contrary, electron transport in crystalline wires is severalorders of magnitude faster than in the random polycrystallinenetwork [15]. In addition, dense arrays of long and thin NWsincrease the overall surface area and the dye loading while si-multaneously maintaining very efficient carrier collection.

A typical layout of a ZnO-NW-based DSSC is presented inFig. 7. Under illumination, the photons absorbed by the dyemolecules produce excitons, which rapidly split at the surfaceof the NW. The electrons are then injected into the NWs, whileholes leave the opposite side of the device with the help ofredox species in a liquid or solid electrolyte. An overall energyconversion efficiency of 1.5% has been demonstrated by Lawet al. [15].

However, the efficiency is far below that of TiO2 NP-basedDSSCs, where ∼10% efficiency has already been demonstrated[80]. This is mainly due to the larger surface area of the NP-based device. In addition, TiO2 NPs offer superior chemicalstability than ZnO. In case of ZnO-NW DSSCs, the acidic na-ture of the dye may cause etching of the surface. To get rid ofthese limitations, an interesting architecture has been proposedby Leschkies et al. [16], where the dye was replaced by semicon-ductor quantum dots in the usual design. This architecture hasthe advantage of tunability of optical absorption through selec-tion of the semiconductor material and the particle-size distribu-tion. In addition, the device can exploit multiple electron–holepair generation per photon [81] to achieve higher efficiency.Very recently Wei et al. [18] introduced a new 3-D DSSC byalternatingly sandwiching quartz slides covered with alignedNW arrays with planar electrodes. The ZnO NWs were grownnormally to both surfaces of the quartz slide, which serves as aplanar waveguide for light propagation. Each time when lightreaches the waveguide–NW interface, photons are coupled intothe ZnO NWs and then are absorbed by the dye molecules. Onaverage, an enhancement of the energy conversion efficiency bya factor of 5.8 has been achieved when light propagating inside

Fig. 8. Coupling of green laser light from a silica-tapered fiber into a ZnONW (a) from one end and (b) at the middle. Image source: Image reproducedwith permission from [84].

the slide is compared to the case of light illumination normal tothe surface.

D. ZnO-NW Waveguides

The confinement of the light field in structures much smallerthan the wavelength is crucial to realize integrated photonicdevices for communication, computing, and sensing. Due tothe large refractive index of ZnO across the visible spectrum(n > 2) [82] and large bandgap at room temperature, ZnONWs are excellent low-loss subwavelength waveguides. Theyprovide tight optical confinement for low-order modes and largeacceptance angles for light coupling. Although the hexagonalcross section of the NWs influences the intensity profile ofhigher orders modes, the lowest order mode exhibits almostcircular symmetry inside the NW [83].

The waveguiding properties of single ZnO NWs have beeninvestigated by coupling external light into the wire with taperedsilica fibers [84], [85] or SnO2 NWs [6]. A typical couplingprocess between a silica-tapered fiber and a ZnO NW is shownin Fig. 8. The taper region can be brought close to one end ofthe wire [see Fig. 8(a)] or close to the center region of the NWsuch that the two waveguides form an angle of about 90◦ [seeFig. 8(b)]. In the latter case, the light is guided to both ends of theZnO NW from where it is emitted. While significant scatteringis observed in the coupling region, no additional waveguidinglosses along the ZnO NW are observed. The light is guided tothe opposite end of the wire where it is emitted. The angle ofemission from a NW with a typical diameter of about d = 200nm was found to be approximately α = 90◦ [85]. This result


Fig. 9. Diameter and wavelength dependence of the fraction of power guidedin the lowest order HE11 mode inside a cylindrical ZnO NW. The remaining partof the power is guided in the evanescent field. The crosses show the transitionbetween the single-mode and multimode waveguiding regimes (single-modeguiding occurs for diameters smaller than that at the cross). Image source:Image reproduced with permission from [22].

was confirmed in numerical simulations based on the finite-difference time-domain technique.

The diameter of the analyzed VLS-grown ZnO NWs typicallyvaries between 100 and 400 nm. For cylindrical waveguides inthis diameter range, both single- and multimode waveguiding isexpected from waveguide theory [22]. The single-mode cutofflimit can be estimated with the V number defined as

V = (2π/λ0)d√

(n2ZnO − n2

air)

where λ0 is wavelength in vacuum and n is the refractive index.If the V number is smaller than roughly 2.4, the correspondingwaveguide supports only a single mode [22]. The dependenceof the guided power of the lowest order HE11 mode on the wirediameter and the wavelength is shown in Fig. 9. The cross marksthe transition from the single-mode to multimode waveguiding.The transition occurs when the power guided in the ZnO NWreaches about 80%. The results show that for light in the visiblespectral region, the transition from multimode to single-modewaveguiding occurs for ZnO NW diameters between 150 and300 nm.

This is the typical diameter range for VLS-grown ZnO NWs,which thus may act as either single-mode or multimode waveg-uides depending on their actual diameter. Multimode waveguid-ing in ZnO NWs with diameters of about 250–300 nm hasalso been studied by exciting the corresponding waveguidemodes using the silica-tapered fibers. A slight change in thealignment between the silica-tapered fiber and the ZnO NWwaveguide leads to the excitation of significantly differentwaveguide modes. The high-order modes possess additionalevanescent-field contributions, which exponentially decreasewith the radial distance from the NW surface. These evanes-cent components are very sensitive to roughness and impuritiesin the substrate, and thus experience noticeable scattering lead-ing to a green appearance of the ZnO NW surface. Additional

Fig. 10. Schematic of the gold-NP-terminated ZnO NW waveguide that isscanned on a cell membrane conjugated with gold NP SERS or FL probes.Image source: Image reproduced with permission from [88].

investigations with a white-light source instead of the greenlaser have demonstrated that the high-order waveguide modeswith strong evanescent contributions experience wavelength-dependent losses. This is expected because the penetration depthof the evanescent mode into the surrounding medium increaseswith the wavelength. The white-light waveguiding measure-ments, therefore, support the interpretation of the experimen-tally observed effects as being due to high-order waveguidemodes with significant evanescent-field contributions.

The high-order waveguide modes with their strong evanes-cent fields can be used to develop optical sensors for monitor-ing pollutant gases and their concentrations using the interac-tion of an evanescent wave with the ambient gas. Especiallythe near-infrared (NIR) spectral region is attractive, since vi-brational or vibrational–rotational transitions of oxygen (O2),carbon monoxide (CO), or carbon dioxide (CO2) can be stim-ulated and, therefore, detected by NIR spectroscopy. A highsensitivity is conventionally realized by optical fibers, where abare core acts as sensing element; here, rather long fibers witha large number of internal reflections are essential for efficientinteraction [86]. Alternatively, the sensitivity can be increasedby adequate chemical sensitive layers [87]. Here, NW-basedevanescent-field optical sensors may provide a higher sensitivitywhile at the same time reducing the size and power consumptionof the sensing element.

Recently, a very interesting experiment for molecular imag-ing by utilizing waveguiding properties of ZnO NW has beenproposed and theoretically studied [88]. Fig. 10 illustrates theschematics of the proposed device that contains a transparentSiNx/SiO2 cantilever covered by a 50-nm-thick metal film. AVLS transport process can be used to grow a ZnO NW at theend of the cantilever from a ∼100 nm diameter opening pat-terned into the metal film. The obtained VLS ZnO NW is self-terminated with a catalytic gold NP at the free end [89]. Whena laser beam is focused through the transparent cantilever ontothe junction between the ZnO NW and the cantilever, the metalfilm on the cantilever will block the laser beam except for thatpart, which is coupled into the ZnO NW. When the NW with


the gold NP at the tip is scanned across a gold substrate or agold NP on a substrate, the local electric field is enhanced dueto plasmon coupling. The region of field enhancement is aboutthree times smaller than the 100 nm diameter of the tip, makingthe plasmon tip well suited for molecular imaging.

The plasmon coupling can be used for imaging moleculesimmobilized on a gold substrate or gold NP surface-enhancedRaman scattering (SERS) or fluorescence (FL) probes conju-gated on a live cell membrane. For the latter case, plasmoncoupling between the gold tip and a gold NP probe will exciteSERS or FL from fluorescent or Raman tag molecules conju-gated to a gold NP probe on a live cell membrane [90]. Thehighly localized field enhancement can be used to obtain sub-50-nm resolution. In addition, the high refractive index of theZnO NW waveguide allows for tight light confinement, evenin liquid environments. This feature will allow for imaging ofbiologic cells and molecules in their natural liquid environmentby using the gold tip-terminated NW waveguide operated intapping-mode atomic force microscope [91].

E. Nonlinear Optics

Being a highly polar semiconductor, ZnO is often used forfrequency doubling of intense ultrashort laser pulses [23]–[28].Because of the high degree of c-axis orientation of ZnO NWs,densely packed NW arrays are promising candidates for efficientnonlinear optical devices. Since the symmetry of the crystalstructure is broken at the NW surface, the surface can even beexpected to additionally increase the nonlinear optical responseof NWs compared to that of larger bulk crystals. Coating the NWsurface with additional organic and inorganic compounds shouldallow for a tailoring of the strength of the nonlinear signal,thereby enabling the design of efficient nanoscale nonlinearoptical sensors.

The optical nonlinear response of ZnO NWs has been inves-tigated widely in the past decade [24]–[28]. Using near-fieldscanning optical microscopy technique Johnson et al. [23] de-termined the magnitude of the two independent χ(2) elementsnamely χ

(2)zzz and χ

(2)zxx of a single NW. The investigation showed

that the NW second-harmonic generation (SHG) and third-harmonic generation (THG) emission pattern strongly dependon the polarization of the incident beam due to the anisotropy ofthe wire. The approximate magnitude of each χ(2) componentas determined by using ZnSe disk (78 pm/V) as a reference wasχ

(2)zzz = 5.5 pm/V and χ

(2)zxx = 2.5 pm/V. The value of χ

(2)zzz is

considerably lower than the reported bulk value (18 pm/V), butin relatively good agreement with values reported for ZnO thinfilms (4–10 pm/V). One of the possible reasons for the lowervalue compared to bulk ZnO is that the amount of materialprobed for a single NW is less than that probed on a solid disk.The maximum SHG signal for each wire was found to occurwhen the linear polarization was aligned along the NW symme-try axis, in which case χ

(2)zzz is probed. This is in good agreement

with the theoretical predictions that suggest that SHG cannot beefficiently generated with the incident-beam propagation direc-tion exactly parallel to the symmetry axis of the NW [92]. Incontrast, efficient THG could be realized by pumping the NW

Fig. 11. Contour plot showing light emission intensity as a function of pumpemission wavelength. Structures due to SHG and multiphoton PL are marked.Image source: Reproduced with permission from [24].

Fig. 12. Emitted intensity is shown as a function emission wavelength from710- and 1000-nm pump light. The 710-nm data have been fitted with twoGaussian contributions. Image source: Reproduced with permission from [24].

along the symmetry axis. This is because several χ(3) compo-nents can contribute to the third-order response, resulting in astrong THG signal.

The wavelength dependence of the SHG intensity from ZnONWs was investigated by Pedersen et al. [24] using a series ofpump wavelengths between 710 and 1000 nm. A contour plotof these data is shown in Fig. 11.

Two structures forming straight lines in the figure are seenwith the highest intensity from the SHG signal at half the pumpwavelength, while the weaker horizontal structure results frommultiphoton-excited luminescence close to the ZnO bandgap.

The PL, appearing at shorter wavelengths than the SHG (be-low the SHG line), is presumably three-photon-excited lumi-nescence, while the much stronger two-photon process is seenabove the SHG line. Fig. 12 shows scans taken at pump wave-lengths of 710 and 1000 nm, respectively. It can be seen thatat 1000 nm, the SHG signal dominates completely, while at710 nm, a relatively broad emission from the band edge domi-nates the signal though the SHG signal is clearly seen.

Voss et al. [19] performed transmission experiments on ZnO-NW waveguides to study the coupling efficiency of both thesecond-harmonic and the multiphoton-induced PL into the NWwaveguide modes. By comparing the emission spectra at theinput and output of the NW, it was observed that the PL andthe second harmonic couple into the waveguide mode with


distinctly different efficiencies. Due to the isotropic nature ofthe PL, it can efficiently couple into the waveguide mode ascompared to the second harmonic, which is mainly generated inthe direction normal to the axis of the NW. In this experiment,a single-NW transmission spectrum was obtained by normaliz-ing the transmitted intensity by the input intensity. The authorsfound a clear red shift of the ZnO bandgap of about 150 meV inthe NWs under excitation with femtosecond laser pulses. Theyattributed this effect to local laser heating, since the heat dissi-pation from the isolated NW lying on a transparent mesoporoussilica substrate was rather inefficient. The authors demonstratedin numerical simulations that a temperature increase of 300 Kis a reasonable assumption under their experimental conditions.The experiment showed the potential of nonlinear optical exci-tation of ZnO NWs for investigating their waveguide propertiesand determining their absorption spectrum.

F. ZnO NW Lasers

One of the most important and widely investigated applica-tions of ZnO NWs is optically pumped ultraviolet lasing. Sincethe first report by Huang et al. [29], many research groupshave studied the mechanism of optically excited lasing in ZnOmicro- and nanostructures including micropillars, NWs, andnanorods [30]–[34]. High-quality single-crystalline ZnO NWsconstitute ideal lasing nanocavities, which provide both a gainmedium and a resonant cavity due to reflection at the planarend-facets [35], [36]. The strong optical confinement that oc-curs due to the large index difference between the NW and thesurrounding material provides a large modal gain by maximiz-ing the overlap between the guided optical mode and the gainmedium.

Fig. 13 shows the evolution of the luminescence of a singleZnO NW, as the excitation power is increased [34]. Spectrallysharp emission lines appear above a certain threshold value[see Fig. 13(a)] that evolve from the low-energy side of theluminescence spectrum at each stay fixed at a certain spectralposition.

In Fig. 13(b), the spatial intensity distribution is shown incolor-coded maps for increasing excitation density. For com-parison, the SEM image of the analyzed NW is also given.Whereas for the lowest two excitation densities, the lumines-cence is homogeneously emitted from the whole NW (as ex-pected for isotropic PL), the intensity emitted from the NWends substantially increases for excitation densities well abovethe lasing threshold. This is a clear indication of stimulated emis-sion and lasing from a single ZnO NW, which was confirmed bythe authors by also studying the integrated output intensity asa function of the excitation power. As expected for laser emis-sion, the emission power still increased linearly above the laserthreshold, however, with a significantly larger slope [34].

The authors furthermore studied the length and diameterrange of ZnO NWs for which they could observe lasing emis-sion. They found that only wires with diameters d > 150 nm andlengths l > 5 μm showed laser emission above a certain thresh-old density. The authors explained this observation by 1) thelimited round-trip gain that needs to overcome the losses due

Fig. 13. Laser oscillation in ZnO NWs. (a) Output spectra versus pump in-tensity of a 12.2-μm-long 250-nm-diameter ZnO NW. (b) Scanning electronmicroscopy and charge-coupled device (CCD) images under different pumpintensities for the same NW as in (a). The labels indicate the pump intensityin units of MW/cm−2 . The color scale indicates the number of counts. Imagesource: Figure reproduced with permission from [34].

to the relatively low reflectivities at the NW facets for smalllength and 2) the strong evanescent fields that emerge outsidethe wire for small diameters. The evanescent-field contributionsno longer overlap with the NW gain medium, but experience ad-ditional scattering losses in the substrate. This prevents the NWfrom providing enough modal gain to overcome the losses for theoptical mode during one roundtrip through the cavity. Togetherwith the experiments discussed in Section III-D about ZnO-NWwaveguides, these results provide important information aboutthe efficient design of NW nanolasers, especially when the goalis to achieve electrical operation, each parameter of the devicewill need a careful and precise optimization to ensure maximumefficiency and optimized lifetime of the nanolaser.

Lasing in single isolated NWs was also investigated by John-son [33], Gargas [93], and others [20], [94], [95]. Fig. 14(a)shows an SEM image of vertically oriented ZnO NWs, whichare well separated from each other. Three different longitudi-nal Fabry–Perot lasing modes (λA , λB , and λC ) of a ZnO NWcavity are schematically illustrated in Fig. 14(b).

The Fabry–Perot modes of a single ZnO vertical NW cavityare observed in PL spectra collected at increasing pump ener-gies, as shown in Fig. 14(c). The PL spectra (vertically offsetfor clarity) were collected at pump energy densities (Pex) of383, 536, 980, 1378, 1454, and 1760 μJ/cm2 . For Pex less than400 μJ/cm2 , the spectra are broad and featureless with a PL peakwidth of 15.9 nm [full-width at half-maximum (FWHM)], whichis consistent with spontaneous emission. However, at pump en-ergies above 400 μJ/cm2 , the PL peak width decreases sharplyto ∼0.8 nm and the intensity increases significantly above thespontaneous-emission background. This transition from sponta-neous to stimulated emission transition in PL emission is clearlyseen in a plot of total PL intensity versus Pex (see Fig. 14(c) insetgraph). The inset images in Fig. 14(c) show a single ZnO verticalNW cavity under white-light illumination (top) and stimulatedemission (bottom).


Fig. 14 (a) SEM image of ZnO vertical NW cavities grown on sapphiresubstrate. (b) SEM image of single vertical NW with diagram showing Fabry–Perot lasing modes as wavelengths λA , λB , and λC . Scale bar is 300 nm. (c)Lasing spectra of single ZnO vertical NW cavity. Left inset: power dependencegraph showing lasing threshold at roughly 400 μJ/cm2 . Right inset: dark-fieldscattering images of a ZnO vertical NW cavity from white-light illumination(top) and lasing induced by 266-nm pulsed excitation (bottom). Scale bar is2 μm. Image source: Image reproduced with permission from [93].

Lasing action in ZnO NWs is mainly determined by theFabry–Perot cavity modes. Well-faceted NWs with diametersof 100–500 nm support predominantly axial modes separatedby Δλ = λ2/[2Ln(λ)], where L is the cavity length and n(λ) isthe index of refraction. The typical output power of an opticallypumped, single ZnO NW laser was recently measured to be sev-eral tens of microwatts [34]. However, in contrast to the highlydirectional output of macroscopic laser sources, the analysis ofthe interference pattern [36] from the wire end-facets suggeststhat the ZnO NW laser emission might be nondirectional.

IV. CONCLUSION

During the past decade, a tremendous progress has beenachieved in research involving ZnO NWs. It has been found tobe an extremely suitable material for optoelectronic applicationswith many different functionality, such as sensing, switching,light emission, lasing, waveguiding, and nonlinear optical mix-ing. However, there are still fundamental issues that need to beresolved. Although, the state-of-the-art synthesis techniques are

capable of fabricating NWs with exceptionally good crystallinequality, the external efficiency of luminescence from these NWsare still very poor because of high density of intrinsic defectsand nonradiative channels. In addition, despite many decades ofresearch, the p-doping in ZnO still remains a challenge. Never-theless, considering the huge potential, it is expected that oncethese challenges are met, ZnO NWs surely can revolutionize thefuture of nanoscale optoelectronics.

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