Multiangular Branched ZnS Nanostructures with Needle-Shaped Tips

J. Phys. Chem. C 2008, 112, 4735-4742

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Multiangular Branched ZnS Nanostructures with Needle-Shaped Tips: Potential Luminescent and Field-Emitter Nanomaterial Xiaosheng Fang,*,† Ujjal K. Gautam,*,† Yoshio Bando,† Benjamin Dierre,‡ Takashi Sekiguchi,‡ and Dmitri Golberg† Nanoscale Materials Center and AdVanced Electronic Materials Center, National Institute for Materials Science, Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan ReceiVed: December 6, 2007; In Final Form: January 9, 2008

A facile and effective route toward the synthesis of ZnS multiangular branched nanostructures with needleshaped tips has been developed. An average width of the structurally and chemically uniform, single-crystalline, defect-free tips was found to be less than 10 nm. Cathodoluminescence from individual ZnS multiangular branched nanostructures was investigated at high spatial resolution on the nanometer scale. The size-dependent optical spectra exhibit sharp ultraviolet band gap emission and broad visible emission. Field-emission measurements show a relatively low turn-on field of ∼3.77 V/µm at a current density of 10 µA/cm2 and the highest field enhancement factor β of ∼2182 ever reported for 1D ZnS nanostructures. This was attributed to the specific sharp tips and high aspect ratios. These unique nanostructures are envisaged to be highly promising for novel optoelectronic and field-emitting devices.

1. Introduction Understanding the fundamental physical properties of novel nanomaterials is essential for the rational development and optimization of functional devices. It is also of prime importance for the design of completely new nanoscale device concepts in electronics, optoelectronics, and other areas.1-10 Cathodoluminescence (CL) is an optoelectrical phenomenon wherein a beam of electrons is generated by an electron gun and interacts with a luminescent material such as phosphor, causing the material to emit visible light. It is a very useful technique for the characterization of nanostructure optical properties.11 In addition, spatially resolved CL on a single nanostructure may provide useful information. Field emission (FE) is based on the physical phenomenon of quantum tunneling, during which electrons are injected from a material surface into vacuum under the influence of an applied electric field.12 Since the discovery of carbon nanotubes (CNTs), much attention has been paid to explore the prospects of one-dimensional (1D) nanostructures as fieldemitters because of their low work functions, high aspect ratios, high mechanical stabilities, high conductivities, and so forth. ZnS is one of the first semiconductors discovered and probably one of the most important materials in electronics with a wide range of applications.13 Its nanostructures have been proven to be ideal systems for exploring a large number of novel phenomena at the nanoscale and investigating the property-size dependence.13-17 Although much research has been devoted to the optical properties and applications of 1D ZnS nanostructures, very few studies have been reported on their possible ultraviolet band gap emission at room temperature (RT). This is mainly due to high sensitivity of the 1D ZnS nanostructure optical properties to the synthetic conditions, its crystal size and shape,18 * To whom correspondence should be addressed. E-mail: Fang. [email protected] (X. S. Fang); [email protected] (U. K. Gautam), Fax: (+81) 29-851-6280. These authors contributed equally to this work. † Nanoscale Materials Center. ‡ Advanced Electronic Materials Center.

and some intrinsic defects such as vacancies and interstitials, which may form additional energy levels within the ZnS band gap, for example, donor levels of VS and Zni, acceptor level of VZn, and so forth.19 Therefore, it is of special interest to explore and control the ZnS ultraviolet band gap emission. With respect to FE applications, it is well known that the needle-shaped tips can enhance material FE properties significantly.20-22 Although various ZnS 1D nanostructures have been synthesized successfully, ZnS nanostructures with needle-shaped tips have never been reported to our knowledge so far. In this article, we developed a facile and effective route toward the synthesis of ZnS multiangular branched nanostructures with needle-shaped tips. The average width of the tips was less than 10 nm. Each tip was composed of chemically pure, structurally uniform, single-crystalline, and defect-free ZnS. These features made the observation of ultraviolet band gap ZnS emission at RT possible. The present article is also devoted to high-spatial-resolution CL and FE studies of such structures. RT CL spectra along a needle-like tapered branch with decreasing diameter exhibited sharp ultraviolet band gap emission and broad visible emission and size-dependent optical properties. FE measurements show that the nanostructures possess good FE characteristics with a relatively low turn-on field of 3.77 V/µm at a current density of 10 µA/cm2 and the highest field-enhancement factor β of ∼2182 among all previously reported 1D ZnS nanostructures. With perfectly crystalline needle-shaped tips, the as-synthesized ZnS nanostructures may become an attractive nanomaterial for new optoelectronic and FE nanoscale devices. 2. Experimental Section The synthesis of ZnS multiangular branched nanostructures with needle-shaped tips was conducted in a conventional horizontal tube furnace with a 36 mm inner-diameter quartz tube. A long alumina plate or a quartz piece (30 cm in length) was placed to act as the deposition substrate. High-purity commercial ZnS powders (∼10 µm, 99.99%) were used as a

10.1021/jp711498m CCC: $40.75 © 2008 American Chemical Society Published on Web 03/05/2008

4736 J. Phys. Chem. C, Vol. 112, No. 12, 2008

Figure 1. XRD pattern recorded from a product.

precursor, which was put into an alumina boat, and the boat was then covered with a quartz sheet to maintain a high vapor pressure. The boat was placed in the center of the tube furnace, and the substrate was placed downstream in the quartz tube subsequently. The tube furnace was purged with high-purity argon (Ar) for 3 h prior to heating to remove any oxygen in the furnace. The detailed experimental parameters are schemed in Figure S1: the furnace was heated to 1050 °C in 9 min, and then the reaction temperature was decreased to 700 °C in 30 min. A constant cooling rate of ∼12 °C min-1 was used to reduce the temperature to a desired value. Then the system was allowed to cool down naturally to RT. High-purity Ar served as a protecting medium and carrying gas, and the Ar flow rate was kept at 100 sccm (standard cubic centimeter per minute) during the entire process. After the system was cooled down to RT, white-colored woollike products were collected from the substrate. The assynthesized products were analyzed and characterized by powder X-ray diffraction (XRD, RINT 2200HF), a field-emission

Fang et al. scanning electron microscope (SEM, JSM-6700F), and a transmission electron microscope (HRTEM, JEM-3000F) equipped with an X-ray energy dispersive spectrometer (EDS). After the structural and chemical examination, spatially resolved CL measurements on individual ZnS nanostructures with needleshaped tips were carried out. CL spectra from individual structures were collected with a high-resolution CL system at an accelerating voltage of 5 kV and a current of 1000 pA at RT by using an ultrahigh vacuum scanning electron microscope (UHV-SEM) with a Gemini electron gun (Omicron, Germany) equipped with a CL system.23 The pressure in the specimen chamber was 10-11 mbar. The present FE measurements were conducted in a vacuum chamber at a pressure of 4.6 × 10-6 Pa at RT. A rod-like aluminum probe with a 1 mm2 cross-section area was used as an anode, and a film composed of ZnS multiangular branched nanostructures with needle-shaped tips served as a cathode. A dc voltage sweeping from 100 to 1100 V was applied to a sample at steps of 5 V. The spacing between the anode and the cathode was set at 200 µm.16 3. Results and Discussion 3.1. Morphology, Structure, and Growth Mechanism of ZnS Multiangular Branched Nanostructures. After the synthesis, white-colored wool-like products were formed onto the substrates. Figure 1 depicts a typical X-ray diffraction (XRD) pattern of a product, where all of the diffraction peaks within an experimental error can be indexed to wurtzite-type ZnS with the lattice constants of a ) 0.382 nm and c ) 0.626 nm (Joint Committee for Powder Diffraction Studies (JCPDS) Card: 361450). A field-emission scanning electron microscopy (SEM) image in Figure 2 shows that the ZnS crystals are composed of multiangular branched nanostructures with needle-shaped tips. Figure 2a and 2b are their low-magnification SEM images, and Figure 2c and 2d shows the corresponding high-magnifica-

Figure 2. SEM micrographs of the as-grown ZnS nanostructures, showing that they are composed of multiangular architectures with needleshaped tips. (a and b) Low-magnification SEM images. (c and d) High-magnification SEM images verifying the multiangular and needle-shaped structures.

Multiangular Branched ZnS Nanostructures

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Figure 3. TEM images of ZnS nanostructures with various shapes. (a) A biangular structure, (b) a triangular structure, (c) a tetra-angular structure, and (d) another triangular structure. All arm images reveal needle-shaped tips.

tion SEM images. The diameter of each nanostructure needleshaped branch decreases along its length from its center toward its tip. An overall triangular ZnS nanostructure is shown in Figure 2d. Each arm has a straight appearance with a needle length reaching several micrometers. Statistics based on numerous TEM observations (more than 100 nanostructures) indicates that the multiangular branched ZnS nanostructures have been composed of mainly biangular structures (∼20%), triangular structures (∼50%), tetra-angular structures (∼20%), and some other multiangular nanostructures (∼10%). Besides them, some individual ZnS nanowires with needle-shaped tips or their assemblies were observed. The average width of the sharp tips was ∼8 nm. Figure 3 illustrates nanostructures of different morphologies all having sharp needlelike tips as a common characteristic. Further investigations were carried out employing highresolution transmission electron microscopy (HRTEM), electron diffraction (ED), and X-ray energy dispersive spectrometry (EDS). Figure 4 displays TEM and HRTEM images, ED pattern, and EDS spectrum of the single arm within a multiangular nanostructure. The length of the arm, shown in Figure 4a, reaches several hundred nanometers. HRTEM images of its tip and stem suggest that its surface is rather clean and atomically sharp; no dislocations or stacking faults are observed along the arm length. Each arm grew along the same direction, which is [001]. The EDS spectrum, shown in Figure 4d and taken from

the stem, confirms that it consists of Zn and S with a stoichiometric ZnS composition. The Cu peaks come from a TEM grid. The similar spectra were obtained in various spots on the arms and their tips. Two mechanisms have been proposed to account for the growth of 1D nanostructures by the thermal evaporation method, namely, vapor-liquid-solid (VLS)25 and vapor-solid (VS) mechanisms.6 We suggest that the VS mechanism might dominate the growth of the present unusual structures because only high-purity commercial ZnS powders were used as a precursor. The ZnS vapor is rapidly generated at a relatively high-temperature (∼1050 °C) through the evaporation of ZnS powders, transported by carrying gas (Ar) to a relatively lowtemperature zone, where it condensates in the form of 1D nanostructures. Here we utilized a gradual reduction in heating temperature from 1050 to 700 °C over 30 min. Recently, the formation mechanisms of multiangular inorganic nanostructures have become of special interest. With respect to ZnO tetrapod nanostructures, the most successful model has been proposed by Takeuchi et al., that is, the octa-twin model, which shows a good agreement with the crystallographic studies.25 Some recent experimental results on multiangular ZnO, ZnS, and ZnSe nanostructures have further verified this model.26 We believed that this model is also applicable in the case of the present multiangular branched ZnS nanostructures. However, the notable thicknesses or the junction segments (∼200 nm), Figure 3, do

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Figure 4. (a) TEM image of the individual arm of a multiangular branched nanostructure. (b and c) The corresponding HRTEM images of its tip and stem, respectively. The inset in c shows the corresponding ED pattern. (d) EDS spectrum taken from the arm, shown in a.

not allow us to verify this hypothesis experimentally using HRTEM lattice imaging. 3.2. Spatially Resolved Cathodoluminescence (CL) from Individual Nanostructures. ZnS is one of the most promising candidates for future optical and electrical applications, par-

Fang et al. ticularly in electroluminescence, nonlinear optical devices, biological markers, and so forth. Spatially resolved CL is one of the most effective methods to explore the optical properties of a nanomaterial. In the present work, the optical properties of individual multineedle ZnS nanostructures dispersed on standard C-coated TEM copper grids were studied with high spatial resolution in SEM. The research on ZnO nanomaterials has expanded rapidly in recent years, and the band-edge emission (UV region, ∼373390 nm) at RT has been observed.27 Although ZnS and ZnO have very similar fundamental properties,14 such RT band gap emission in 1D ZnS nanostructures has not been evident. One of the main reasons for this is that intrinsic defects, such as vacancies and interstitials, may form new energy levels in ZnS hindering the natural band gap emission. In the present work, we developed a facile and effective route to enhance the band gap emission from the 1D ZnS nanostructure at RT. Figure 5b depicts a typical CL spectrum obtained at RT from an individual ZnS multiangular branched structure. The corresponding SEM image is shown in Figure 5a. The CL spectrum is composed of a broad visible emission band centered at ∼404 nm and a sharp ultraviolet emission band centered at ∼334 nm. The inset in Figure 5b is the enlarged portion of the sharp ultraviolet emission band. The intensity ratio between the ultraviolet emission and visible emission is ∼1/10. Figure 5c and d shows CL images of the same nanostructure recorded at 334 and 404 nm, respectively. Similar CL spectra with hardly noticeable intensity and peak position differences were collected in several individual structures. Figure 6a and b shows a SEM image of another structure and its RT CL spectrum, respectively. In the latter case, the intensity ratio changes from 1/10 to 2.2/ 10 and the peak positions are slightly shifted (∼1 nm). Then the CL spectra were collected in different spots along the individual tapered arms of varying diameter. An example is shown in Figure 7. Figure 7a displays a SEM image of the left-hand-side part of the ZnS multangular branched nanostructure in Figure 6a. The RT CL spectra (Figure 7c) denoted as C1, C2, C3, C6, C9, C12, C15, C16, and C17 were taken in different spots between the two red lines in Figure 7b. The CL ultraviolet emissions are very strong and sharp in some spots. The centers of the ultraviolet emissions shifted gradually from 337 to 333 nm (∼0.043 eV blueshift) with diameter descending, while the broad visible emissions change between 398 and 425 nm. It is assumed that the relatively sharp CL ultraviolet emission centered at ∼333 to 337 nm represents the band-edge luminescence, suggesting a band gap energy of ∼3.7 eV for the present wurtzite-type ZnS nanostructures. The small blueshift in ultraviolet emissions is referred to as the “anomalous” blueshift,28 which does not arise from the quantum confinement because the diameter of a ZnS arm is far larger than its exciton Bohr radius (∼5 nm). The CL ultraviolet emission from the samples was very stable against a prolonged exposure to air and from one measurement to another. The CL spectra remained unchanged being consecutively recorded within a period of nearly 1 year. For further proving our assumption that the ultraviolet emission peak may be derived from the band edge luminescence, we offer more results on different individual multiangular branched ZnS nanostructures that shows that the centers of the ultraviolet emissions have little shift. The results were shown in the Supporting Information (Figures S2-3). In the meantime, more extensive work to analyze the ultraviolet emission on other ZnS nanostructures is underway, including low-temperature CL and so forth.

Multiangular Branched ZnS Nanostructures

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Figure 5. (a) SEM image of a ZnS multiangular branched nanostructure with needle-shaped tips. (b) CL spectrum recorded at RT from the structure shown in a. The inset shows the enlarged spectrum in the band gap region. (c and d) CL images of the same nanostructure recorded at 334 and 404 nm, respectively.

TABLE 1: Comparison of the Most Important ZnS Field-Emission Parameters between This Work and the Previous Reports in the Literaturea

ZnS emitters nanobelts or nanoribbons

nanowires nanorods needle-shaped tips

synthesis method

turn-on field (V/µm)

fieldenhancement factor (β)

reference

CVD

3.47

2010

16

thermal-evaporation process solvothermal reaction vapor-phase deposition radio frequency magnetron sputtering technique thermal-evaporation process

3.55

1850

17

3.8 11.7 at 0.1 µA/cm2 2.9-6.3 for different diameter at 2.452 µA/cm2 3.77

1839 522 420-105 for different diameter 2182

30 31 32 this work

a We define the turn-on field at a field producing emission current density of 10 µA/cm2. If the other values are used, then this is mentioned separately.

In recent years, many researchers have reported visible emissions in 1D ZnS nanostructures using photoluminescence (PL) and CL. The reported visible emissions centered mainly at 430-550 nm and presumed the existence of some selfactivated centers, for example, vacancy states, elemental sulfur species on the surface, interstitial states, and so forth. For example, Ye et al. reported a PL band centered at ∼535 nm in pure wurtzite ZnS nanobelts. The authors proved that the origin of this intense PL had been related to elemental sulfur species on the belt surfaces.19 Yin et al. reported that pure ZnS nanotubes had exhibited two emissions at 538 and 439 nm. The authors assumed that the emissions were caused by a large surface-to-volume ratio and the stoichimetric defects.18 Jiang et al. reported a weak band gap emission and a strong green emission centered at ∼340 and 515 nm, respectively. The luminescence peak at 515 nm was attributed to the recombina-

tion of electrons (from S energy level) with holes (from Zn vacancies level) within the band gap.14 Recent research on sizedependent luminescence of ZnO nanowires or nanorods indicates that strong surface modulation can become dominant with a decrease in nanostructure diameter, and a given structure possesses a remarkable linear increase in intensity ratio between the deep level (IDL) and the near band edge (INBE) with an increase in its surface-aspect ratio. The intensity ratio of IDL/ INBE increases by an order of magnitude with the diameter decreasing and DL emission becomes more dominant at spots located in smaller diameter portions.29 However, from Figure 7c, an intensity ratio between the ultraviolet emission (∼333 to 337 nm) and the emission at ∼398 to 425 nm is kept stable within the ratio of 1-2 and the two increase or decrease simultaneously. Moreover, the intensity ratio almost did not change when the measurements were performed at low tem-

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Fang et al.

Figure 6. (a) SEM image of another ZnS multiangular branched nanostructure and (b) the corresponding CL spectrum recorded from it at RT. The inset in b is the enlarged part of the emission band centered at ∼333 nm.

perature (∼30 K) (not shown here). Therefore, the origin of the present broad visible emission bands (∼398-425 nm) is more complex and requires further studies. 3.3. Field-Emission (FE) Properties of ZnS Multiangular Branched Nanostructures. FE is one of the main features of nanomaterials that is highly valuable in displays and other electronic devices.12 In general, it is established that a FE material with a lower work function (φ) can produce a higher electron emission current (I) or current density (J) at a specific field (E). The work function for ZnS (7.0 eV) is higher than that for some other popular FE materials, such as C nanotubes (5.0 eV), ZnO (5.3 eV), and so forth. As a rule, ZnS is not among the best FE materials. Very recently, we have developed two alternative methods to improve the FE properties of ZnS nanostructures dramatically: the first one is to increase their aspect ratios through making ZnS ultrafine nanobelts with narrow size distribution;16 the second one is to fabricate crystal orientation-ordered ZnS nanobelt quasi-arrays using a noncatalytic and template-free thermal evaporation process.17 In the present work, we found that it is possible to further enhance the field-enhancement factor (β) of 1D ZnS nanostructures. The FE current density (J) produced by a given electric field (E) can be expressed by a simplified Fowler-Nordheim (FN) equation12

J ) (Aβ2E2/φ) exp(-Bφ3/2/βE)

(1)

or ln(J/E2) ) ln(Aβ2/φ) - Bφ3/2/βE

(2)

Figure 7. (a) SEM image of the left-hand-side part of the ZnS nanostructure in Figure 6a. (b) The recorded spots were set from C1 to C17 between the two red lines. (c) CL spectra recorded at RT from selective nine spots along the individual arm in b.

where A and B are constants, (A ) 1.54 × 10-6 A eV V-2, B ) 6.83 × 103 eV-3/2 Vµm-1), E is the emitting area, β is the field-enhancement factor, and φ is the work function of an emitting material. The field-enhancement factor, β, is related to emitter geometry, its crystal structure, and the spatial distribution of emitting centers. Figure 8 shows a FE current density, J, as a function of an applied field, E, for a J-E plot (Figure 8a) and a ln(J/E2)-(1/ E) plot (Figure 8b) measured at an anode-cathode separation of 200 µm. From Figure 8a, a relatively low turn-on field is extrapolated as ∼3.77 V/µm at a current density of 10 µA/cm2 and a high field-enhancement factor (∼2182) is calculated from a slope of the fitted straight line in Figure 8b. The emission current density reached ∼3.035 mA/cm2 at a macroscopic field of 5.5 V µm-1. Through the comparison of the present data with all previous works on 1D ZnS nanostructures16,17,30-32 (listed in Table 1) the presently measured β is concluded to be

Multiangular Branched ZnS Nanostructures

J. Phys. Chem. C, Vol. 112, No. 12, 2008 4741 experiments. FE measurements reveal that the present nanostructures are potentially excellent field-emitters with a low turnon field of ∼3.77 V/µm at a current density of 10 µA/cm2 and the highest field enhancement factor β (∼2182) ever reported for ZnS nanostructures. The present low-cost and straightforward method could be employed to synthesize many other new and interesting semiconductor nanostructures valuable for nanoscale optical and electronic devices. Acknowledgment. This work was supported by the Japan Society for Promotion of Science (JSPS) in the form of a fellowship tenable at the National Institute for Materials Science, Tsukuba, Japan (X.S.F.). We also thank Drs. C. H. Ye, P. M. F. J. Costa, Y. Uemura, G. Z. Shen, C. Y. Zhi, C. C. Tang, M. Mitome, and Ms. E. Arisumi for their cooperation and kind help. Supporting Information Available: Schematics of the heating process during the synthesis, typical CL spectra obtained at RT from a representative individual ZnS multiangular branched structure, and CL spectra recorded at RT from 10 selective spots along a specific individual arm. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes

Figure 8. Field-emission properties of ZnS multiangular branched nanostructures with needle-shaped tips. (a) J-E plot and (b) the corresponding Fowler-Nordheim ln(J/E2)-(1/E) plot.

the highest ever reported in ZnS; while the turn-on field still has room for improvement. The highest β values may be interpreted as follows. It is known that β can be expressed as β ) h/r, where h is the height and r is the radius of curvature of an emitting center. Thus, materials with sharp tips/edges would have higher β values. Albeit the possession of highest β, due to a relatively low density of the present ZnS multiangular branched nanostructures compared with our previous works on ZnS nanobelts,16,17 the turn-on field is not the lowest one and the emission current density is not the highest one among the structures studied. 4. Conclusions We have developed a facile and effective thermal evaporation route toward the synthesis of multiangular branched ZnS nanostructures with needle-shaped tips. The growth of ZnS nanostructures with special morphologies could be controlled easily by gradual reducing heating temperature from 1050 to 700 °C over 30 min during the synthesis. Detailed structural characterizations were performed by SEM, TEM, HRTEM, ED, and EDS. They indicate that the average width of the tips is less than 10 nm. Each tip represents a chemically pure, structurally uniform, defect-free ZnS single-crystal. The spatially resolved size-dependent CL spectra at RT exhibit sharp ultraviolet band gap emission and broad visible emission. The ultraviolet emission peak is suggested to represent the bandedge luminescence, whereas the origins of the broad visible emissions are not that clear at the current stage of the

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