IOP PUBLISHING NANOTECHNOLOGY

Nanotechnology 18 (2007) 305705 (6pp) doi:10.1088/0957-4484/18/30/305705

Structure and stimulated emission ofZnSe nanoribbons grown by thermalevaporationFeifei Wang1,2,3, Zhihua Zhang3, Ruibin Liu1,3, Xiao Wang4,Xing Zhu4, Anlian Pan1,3 and Bingsuo Zou1,3,5

1 Micro-Nano Technologies Research Center, and State Key Lab of CBSC, Hunan University,Changsha 410082, People’s Republic of China2 School of Physics and Electronic Engineering, Ludong University, Yantai 264025,People’s Republic of China3 Institute of Physics, Chinese Academy of Sciences, Beijing 100080,People’s Republic of China4 Department of Physics, Peking University, Beijing 100871, People’s Republic of China

E-mail: zoubs@aphy.iphy.ac.cn

Received 16 January 2007, in final form 23 May 2007Published 29 June 2007Online at stacks.iop.org/Nano/18/305705

AbstractZinc selenide nanoribbons were synthesized on silicon wafers by simplethermal evaporation of high purity ZnSe powder. The x-ray diffractionpattern indicates that all diffraction peaks can be attributed to the zinc-blendestructured ZnSe. The selected-area electron diffraction, transmission electronmicroscopy and Raman spectroscopy also show the typical structure of highlycrystalline zinc-blende. The stimulated emission of these ZnSe nanoribbonscan be observed under a nanosecond laser pulse excitation at a threshold ofabout 200 kW cm−2. These ZnSe nanoribbons with more structural defects,which were made from a low purity source, only show an emission band atabout 610 nm in the photoluminescence spectrum. Furthermore, experimentsshow that both types of ZnSe nanoribbons show good waveguide properties.

(Some figures in this article are in colour only in the electronic version)

1. Introduction

Recently one-dimensional nanoscale materials have attracteda great deal of attention because they can act as fundamentalbuilding blocks in many fields [1–5]. Compared withnanowires and nanorods, nanoribbons, with unique geometryand large width-to-thickness ratio, can offer more opportunitiesfor fundamental research and applications [5, 6]. Now allkinds of one-dimensional IV, II–VI and III–V nanomaterialshave been prepared by using different methods [7]. ZnSe,as one of the important functional materials, has a directbandgap of 2.7 eV at room temperature [8]. It is apotential material for applications in blue and short-wavelengthoptoelectronics devices. One-dimensional ZnSe nanomaterialshave been prepared by methods of MOCVD, laser ablation,

5 Author to whom any correspondence should be addressed.

electrodeposition, vapour phase deposition, etc. [9–14]. Inthis paper, we report a simple synthesis of ZnSe nanoribbons,the characterization of structure from different sources and thestimulated emission properties of the sample.

2. Experimental details

ZnSe was synthesized in an electrical furnace with a horizontalquartz tube (35 mm diameter, 100 cm length) by physicalevaporation of ZnSe powder with Au as catalyst. Siliconwafers were first ultrasonically cleaned in acetone and thensputter-coated with thin Au film (10 nm thickness). ZnSe(99.999%) powder was put into the centre of a horizontal tubefurnace, and several clean silicon wafers coated with Au filmwere laid downstream of the gas flow. High purity He wasintroduced into the quartz tube to eliminate the oxygen insideit before heating. After about an hour, the temperature of

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Nanotechnology 18 (2007) 305705 F Wang et al

Figure 1. (a) and (b) Low-magnification and high-magnification images of ZnSe nanoribbons. (c) and (d) XRD pattern and EDS analysis ofZnSe nanoribbons.

the furnace was raised to 1100 ◦C at a heating rate of about25 ◦C min−1 and maintained at this temperature for an hourwhile the high purity He at a rate of 20 sccm was introduced.After that, the electric power was switched off and the furnacewas cooled down to room temperature naturally. Finally,yellow sponge-like products were observed on the siliconwafers, which were placed about 10 cm away from the originalpowder.

The as-synthesized product was characterized by scanningelectron microscope (SEM, Hitachi, S-4200) and high-resolution transmission electron microscope (HRTEM, PhilipsFEG-CM 200). X-ray powder diffraction (XRD) was obtainedby using a Japan Rigaku D/MAX-2400 x-ray diffractometerequipped with a graphite monochromatized Cu Kα radiation(λ = 1.541 78 A). Micro-Raman experiments were conductedin a Raman scattering spectrometer (LABRAM-HR, JY,Horiba), with the 514.5 nm line of an Ar+ laser as theexcitation source. A TU-1901 UV–vis absorption spectrometerand PTI-C-700 fluorescence spectrometer were used to obtainthe reflectance and emission spectra of the sample. A CW He–Cd laser (50 mW, wavelength at 325 nm, IK 3552R-G) wasused to measure the photoluminescence (PL) of ZnSe. Theexcitation power-dependent PL spectra of ZnSe nanoribbonswere excited by a nanosecond (ns) pulsed laser (Nd:YAG(yttrium aluminium garnet), 355 nm, pulse width of 6 ns) anddetected by a spectrometer (Acton Research Corp. Spectra Pro500i) and a liquid N2-cooled CCD camera (Roper Scientific)at room temperature. Optical waveguide properties wereinvestigated using a commercial scanning near-field opticalmicroscope (NSOM) from RHK Technology (USA). Duringthe experiment, the continuous wave laser beam (He–Cd,442 nm; power, 10 mW) was focused on the centre of a singleZnSe ribbon, which was predispersed on a glass substrate. Acolour CCD through an objective lens was used to collect thefar-field optical image of the excited whiskers.

3. Results and discussion

Figures 1(a) and (b) show the SEM images of ZnSenanoribbons at a location where the temperature is about900 ◦C. It was found by the SEM analysis that the productconsists of a large quantity of ribbon-like structures. Thenanoribbons have thicknesses of 20–80 nm, widths of 0.2–2 μm and lengths up to several tens of micrometres.Figure 1(b) shows the typical high-resolution SEM imageof a nanoribbon with a smooth surface, whose thickness isabout 30 nm. The product morphology was sensitive tothe temperature as reported by Dong [15]. Usually hightemperature favours the formation of nanoribbons and lowtemperature favours the formation of nanowires. The XRDspectrum of the sample is shown in figure 1(c). All thediffraction peaks can be indexed to zinc-blende-structuredZnSe within the experimental error (JCPDS: 05-0522), whichis different from that of ZnO [16] and CdS [17]. An energy-dispersive x-ray (EDX) spectrum in figure 1(d) also confirmsthat there are only two elements, which is consistent with theXRD analysis.

The structure and morphology of the product werefurther characterized by HRTEM and selected-area electrondiffraction (SAED). It was found that, though mostnanoribbons have cubic structure, stacking faults and twinlamellae are seen in some nanoribbons.

Figure 2(a) shows a typical TEM image of the nanoribbonswith almost uniform width along the entire length. Thecorresponding HRTEM image and SAED pattern are shown infigure 2(b) and the inset of figure 2(b), respectively. The SAEDpattern can only be indexed to the zinc-blende-structured ZnSewith the incident electron beam parallel to the [011] direction.The growth direction of the nanoribbon is along the 〈111〉direction. It was found by the TEM analysis that almost allnanoribbons are cubic single crystalline in structure.

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Nanotechnology 18 (2007) 305705 F Wang et al

Figure 2. (a) The typical TEM image of the ZnSe nanoribbons. (b) The corresponding HRTEM images and SAED patterns of the nanoribbon.

Figure 3. (a) and (c) The TEM images of mixed phases nanoribbon and twin crystal nanoribbon, respectively. (b) and (d) The correspondingHRTEM images and SAED patterns of the nanoribbon in (a) and (c).

The growth of ZnSe nanoribbons depends strongly onthe precursors. Keeping the other experiment conditionsconstant and replacing the source materials with lower purityZnSe (99.99%), another kind of ZnSe nanoribbon was alsoobtained. The prepared ZnSe has similar morphology as thatprepared from high purity ZnSe (99.999%), but have moredefects in the ribbon as observed by HRTEM. Though theHRTEM observations reveal that most nanoribbons are purecubic structure, other type of structure are observed in somezones. Figure 3(a) is the low-magnification TEM image ofa nanoribbon with two types of structure zones. There are

some obvious dark lines in the ribbon due to bending contoursthat are usually observed by TEM in a slightly bent thinsample [18]. The presence of the two phases in the nanoribbonscan be directly identified by HRTEM. Figure 3(b) shows aHRTEM image from an area in which the hexagonal and cubicphases coexist. The HRTEM image shows clearly that there aresome stacking faults in the zone and a wurtzite phase presentwithin the cubic structure. The corresponding SAED, withtwo sets of diffraction reflections, shows the coexistence of thehexagonal wurtzite structure and the cubic blende structure.The two phases coexist by sharing the same (0001) or (111)

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Figure 4. The microscopic confocal Raman spectrum of high purityZnSe nanoribbons.

plane. The wurtzite structure and cubic structure were formedby changing the stacking sequences of the closed packed planesof the ZnSe crystals.

The wurtzite–zinc-blende (W–ZB) polytypism is commonin compound semiconductors, such as ZnS and ZnSe [19, 20].It was analysed from the structural point that, for a close-packed configuration, a stacking of ABCABC forms a zinc-blende structure, while a stacking mode of ABAB forms awurtzite structure. The (0001) plane is the close packingplane of a wurtzite structure and the (111) plane is the closepacking plane of a zinc-blende structure. So a zinc-blendestructure can be changed to a wurtzite structure by alteringthe stacking sequence of the atomic planes. For ZnSe, thewurtzite structure is stable only at temperatures above 1425 ◦C(100 ◦C below the melting point) and it is transformed to thezinc-blende structure at lower temperatures [21]. Due to thesmall energy difference between the hexagonal and the cubicphase (5.3 meV/atom) [22], a small fluctuation in temperaturemay lead to the coexistence of hexagonal phase and cubicphase in some zones of the sample. From our experiments it isobserved that the impurity in precursors can help the formationof wurtzite phase in a cubic phase.

Apart from mixed phase nanoribbons and single-crystalnanoribbons, there is a small amount of twin-crystalnanoribbons in the low purity sample. TEM image, HRTEMimage and SAED pattern of the twin crystal are shown infigures 3(c) and (d), where the incident electron beam isparallel to the [011] direction. An obvious particle is observedat the end of the typical nanoribbon (the main element is Auproved by the EDS analysis), which indicates the growth ofthe nanoribbons follows the VLS mechanism. It can be seenfrom the inset of figure 3(c) that there are obvious light/darkcontrasts along the length direction. The HRTEM imageindicates that the light/dark contrasts in the low magnificationimage are due to the alternating twins. The existence of thetwins is indicated by SAED. The inset of figure 3(d) showsthere are two sets of spots and each of them corresponds to theZnSe blende structure. The symmetric distribution of atoms inthe HRTEM image and two sets of spots in the SAED imageindicate that the ribbon is a twin-crystal structure.

A microscopic confocal Raman spectrometer was used toobtain the Raman data of ZnSe nanostructures by focusing thelaser facula on different locations. It was found that differentareas show similar Raman profiles. Figure 4 shows the typicalRaman spectrum of a high purity sample. Two Raman peaks

Figure 5. (a) The PL spectrum of high purity ZnSe nanoribbonsexcited by a He–Cd laser and the inset is the reflectance spectrum ofit. (b) The PL spectra of ZnSe nanoribbons excited by the ns laser atdifferent excitation powers and the inset is the changes of emissionintensity and FWHM with the excitation powers.

at 204.63 and 252.75 cm−1 are observed, and these peaks areattributed to the transverse optic (TO) and the longitudinaloptic (LO) phonon modes of a ZnSe (ZB) crystal. There isa shoulder at about 293 cm−1, which may be related to thedefects in nanoribbons. According to previous reports [23–25],the LO phonon frequency of single-crystal ZnSe is 255 cm−1

at room temperature. For ZnSe polycrystalline nanoparticles,the TO and LO phonon frequencies are 210 and 255 cm−1,respectively, and the broad Raman peaks are due to the highsurface-to-volume ratio of small particles. The relativelysharp and symmetric Raman peaks of ZnSe nanoribbons ascompared with the results in [14] indicate that the sample iswell- crystalline, which is also indicated by TEM analysis.

The room temperature PL spectrum was obtained by theexcitation of a He–Cd laser with the wavelength of 325 nm.Figure 5(a) shows a typical room temperature PL spectrum ofthe high purity sample, which exhibits a very strong emissionpeak centred at 468 nm and a weak emission at 480–600 nm.The emission at 468 nm with a full width at half-maximum(FWHM) of about 16 nm is attributed to the near-band-edgeemission, which is associated with the free excitons. Theemission at 533 nm is related to the deep level emissions,which is usually attributed to dislocations, stacking faults andnonstoichiometric defects [26]. The strong near-band-edgeemission and relatively weak deep level emission indicate thegood crystal quality sample with few defects. The inset showsthe reflectance spectrum of this sample, which indicates itsbandgap of about 2.70 eV.

Figure 5(b) shows the excitation power-dependentphotoluminescence spectra of ZnSe nanoribbons excited bythe ns pulsed laser. From the top to the bottom, each curvein figure 5(b) corresponds to the excitation power of 54,197, 289 and 403 kW cm−2, respectively. At low excitationpower, an emission at 471 nm with a FWHM of about15 nm appears, which is attributed to the near-band-edge

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Figure 6. (a) and (b) The far-field image of a ZnSe ribbon with low purity and its corresponding emission images, respectively, when it wasexcited by a focused laser (442 nm). (c) The PL spectrum of the ZnSe ribbons when excited by a laser (325 nm).

emission of ZnSe. The inset of figure 5 shows the changesof emission intensities and FWHM with increasing excitationpower. With the excitation power increases, the near-band-edge emission band turns sharp and increases rapidly.Accompanying with the intensity variation, its FWHM alsobecomes drastically narrower, as illuminated in the inset offigure 5(b). This is due to the coherent transition withoutthermal distribution. The superlinear increase in emissionintensity and the spectral narrowing with increasing excitationpower indicate the presence of amplified stimulated emissionor lasing. The threshold excitation power of this sample isabout 200 kW cm−2.

Figure 6(a) shows the far-field image of a representativelow purity ZnSe ribbon and figure 6(b) is its correspondingemission image (nonintrinsic emission) under the excitation ofa beam of a focused laser. The bright spot in the lower leftof figure 6(b) is the in situ PL image under laser excitation(442 nm); part of the emission was guided through the ribbonand emitted at its ends. It can be seen clearly that the guidedlight mainly is emitted at the end of the ribbon, with a relativelyweak emission in other regions. The high purity ZnSenanoribbons should also show a good waveguide behaviour.Because the wavelength of the pump laser is 442 nm and theband-edge emission of ZnSe nanoribbons is at 468 nm, so theycannot be distinguished on the ends for similar colours. If afilter of 460–700 nm is used, which can filter the light from thepump laser, then the light on the ends can be identified. Ourfilter of 488–700 cannot do this. These results demonstratethat the ZnSe ribbons are good optical waveguide cavities.Figure 6(c) is the PL spectrum of ZnSe ribbons with moredefects. The PL spectrum of ZnSe prepared by low purityprecursors only shows an emission at about 610 nm. This resultindicates that the defects of hexagonal phase may be the sourceof deep level emission of ZnSe.

4. Conclusions

Cubic ZnSe nanoribbons were prepared by a simple physicalevaporation method. The results of XRD pattern, TEM andSAED prove its zinc-blende structure. The PL spectrum ofZnSe nanoribbons excited by a CW He–Cd laser shows astrong near-band-edge emission. The stimulated emission ofthis sample was observed at a threshold of about 200 kW cm−2

with the stimulation of a ns laser pulse at 355 nm. Furthermore,another kind of ZnSe ribbon (mixed phases) was preparedwith lower purity precursors, which possess both cubic andhexagonal phases. The PL spectrum of low purity ZnSenanoribbons only shows a deep level emission at 610 nm,which indicates the defects of hexagonal phase producethe deep level emission. The NSOM observation indicatesthat these ZnSe nanoribbons show good light waveguidingproperties for even low quality nanoribbons. These results maybe useful for future nanophotonic device design.

Acknowledgments

The authors thank the financial support of the NSFC of China(Term no. 90606001, 50602015 and 90406024), National 973Project (2002CB713802), Project (grant no. 705040) of theMOE of China and 985 Fund of the HNU.

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