Extended photoresponse and multi-band luminescence of ZnO/ZnSe core/shell nanorods
© Yang et al.; licensee Springer. 2014
Received: 15 November 2013
Accepted: 5 January 2014
Published: 15 January 2014
Aligned ZnO/ZnSe core/shell nanorods (NRs) with type-II energy band alignment were fabricated by pulsed laser deposition of ZnSe on the surfaces of hydrothermally grown ZnO NRs. The obtained ZnO/ZnSe core/shell NRs are composed of wurtzite ZnO cores and zinc blende ZnSe shells. The bare ZnO NRs are capable of emitting strong ultraviolet (UV) near band edge (NBE) emission at 325-nm light excitation, while the ZnSe shells greatly suppress the emission from the ZnO cores. High-temperature processing results in an improvement in the structures of the ZnO cores and the ZnSe shells and significant changes in the optical properties of ZnO/ZnSe core/shell NRs. The fabricated ZnO/ZnSe core/shell NRs show optical properties corresponding to the two excitonic band gaps of wurtzite ZnO and zinc blende ZnSe and the effective band gap between the conduction band minimum of ZnO and the valence band maximum ZnSe. An extended photoresponse much wider than those of the constituting ZnO and ZnSe and a multi-band photoluminescence including the UV NBE emission of ZnO and the blue NBE emission of ZnSe are observed.
Zinc oxide (ZnO), with a wide band gap (3.37 eV) and a large exciton binding energy (60 meV) at room temperature together with its excellent combined properties [1, 2], is regarded as a promising material in a variety of applications, especially in photoelectronics. Because of its high electron mobility and good chemical stability, ZnO has also attracted much attention for photovoltaic applications [3, 4]. Various ZnO nanostructures, such as nanorods (NRs) and nanowires in particular, are most promising because their properties can be tailored by changing their morphology, structure and size, or modifying their surface with coatings of other materials [5, 6]. Due to its wide band gap, however, ZnO itself can only utilize the light in the ultraviolet (UV) region which accounts for 3% to 5% of the solar energy reaching the earth. Therefore, ZnO has been proposed to form heterojunctions with a narrower band gap semiconductor to extend the spectral region of photoresponse. Zinc selenide (ZnSe), another important Zn-based II−VI semiconductor with a direct band gap of 2.67 eV [7, 8] and its good compatibility with ZnO, has been supposed as an ideal material for ZnO to construct heterojunctions [2, 9, 10].
Aligned ZnO nanorods (NRs) or nanowires are superior to the bulk or film materials in both the surface-to-volume ratio for modifying the surface  and the lateral size for reducing the nonradiative recombination and carrier scattering loss [11, 12]. The modification of surface and interface has been proved to be one of the most advanced and attractive methods to construct novel nanostructures with tailored properties. The surfaces of ZnO NRs can be decorated with ZnSe coatings, constructing the so-called aligned core/shell type-II heterostructures. Compared with the single constituting materials, heterostructures constructed from such nanostructured ZnO and ZnSe can provide better performance when used in photovoltaic process. The band offset between ZnO and ZnSe together with the resulted effective band gap of ZnO/ZnSe core/shell heterojunctions is favorable for improving the transport of both electrons and holes as well as extending the light absorption region to match the solar spectrum. Meanwhile, the staggered band alignment in type-II heterojunctions facilitates the separation of photogenerated electrons and holes, which is an essential procedure in a photovoltaic device and quite significant to enhance the conversion efficiency of solar cells.
In this work, we studied the optical properties corresponding to the respective excitonic band gaps of wurtzite ZnO and zinc blende ZnSe for ZnO/ZnSe heterojunctions in the form of ZnO/ZnSe core/shell NRs. Aligned virgulate ZnO/ZnSe NRs composed of wurtzite ZnO cores and zinc blende ZnSe shells were fabricated by pulsed laser deposition of ZnSe coatings on the surfaces of hydrothermally grown ZnO NRs. The optical properties of the samples were studied by photoluminescence (PL) measurements which show a significant reduction in the emission from ZnO and co-appearance of the near band edge (NBE) emissions of both ZnO and ZnSe. The former suggests the suppression of radiative recombination of photogenerated carriers, while the latter reveals an extended photoresponse which was further confirmed by optical transparency measurement. Both are favorable for photovoltaic applications.
Prior to the growth of ZnO NRs, a dense nanocrystalline ZnO (NC-ZnO) film (approximately 20 nm) was first deposited on a chemically cleaned Si (100) substrate by plasma-assisted pulsed laser deposition. ZnO NRs were grown on the NC-ZnO-seeded Si substrate by hydrothermal reaction. The deposition of NC-ZnO film and the growth of ZnO NRs have been described previously . Serving as the cores, the prepared ZnO NRs were transferred to a vacuum chamber and fixed on a rotating table for the deposition of ZnSe coatings as the shells. The second harmonic of a Q-switched Nd:YAG laser was used to ablate a ZnSe target after being focused by a spherical lens. The laser wavelength, pulse duration, and repetition rate were 532 nm, 5 ns, and 10 Hz, respectively. The focused laser beam with a spot size of 1.2 mm2 was incident on the target surface at an angle of 45°. The laser fluence on the target surface was 2 J/cm2. ZnSe was deposited at a base pressure of approximately 10−4 Pa for 30 min. The deposition of ZnSe coatings were performed at room temperature (RT) or at an elevated temperature of 500°C. The ZnO/ZnSe core/shell NRs obtained by depositing ZnSe at RT were annealed at 500°C in a flowing N2 atmosphere (approximately 105 Pa) for 1 h.
In this paper, bare ZnO NRs without ZnSe shells, as-fabricated ZnO/ZnSe core/shell NRs with RT deposition of ZnSe, as-fabricated ZnO/ZnSe core/shell NRs obtained by depositing ZnSe at 500°C, and ZnO/ZnSe core/shell NRs with RT deposition of ZnSe followed by annealing at 500°C in N2, are named as samples A, B, C, and D, respectively.
Characterization and measurements
The sample morphologies were examined by field emission scanning electron microscopy (FESEM) with a Hitachi S-4800 microscope (Dallas, TX, USA). The crystal structures of ZnO and ZnSe in the samples were characterized by X-ray diffraction (XRD) with a Rigaku D/MAX 2550 VB/PC X-ray diffractometer (Shibuya, Tokyo, Japan) using Ni-filtered Cu Kα radiation (λ = 0.15406 nm). Fourier-transform infrared (FTIR) spectroscopy and Raman scattering spectroscopy were also used to characterize the structures of ZnO and ZnSe through vibrational mode analysis and phase identification. FTIR spectroscopy was carried out with a Bruker Vertex 80 V spectrometer (Saarbrucken, SL, Germany). Raman measurements were performed with a Jobin-Yvon LabRAM HR 800 UV micro-Raman spectrometer (Villeneuve d'Ascq, France) using a 488-nm Ar+ laser beam or 325-nm He-Cd laser beam as the exciting sources. The photoluminescence (PL) of the samples was measured by exciting the samples with 325-nm laser light from a continuous wave He-Cd laser at room temperature to examine the influences of the ZnSe shells on the luminescence from the ZnO cores. The luminescence was detected by an intensified charge-coupled device (ICCD) (iStar DH720, Andor Technology, Belfast, UK) after being dispersed by a 0.5-m spectrometer (Spectra Pro 500i, Acton Research, Acton, MA, USA). The optical properties were also characterized by comparing the optical transparency of ZnO/ZnSe core/shell NRs with that of bare ZnO NRs. The transmission spectra of the bare ZnO NRs and the ZnO/ZnSe core/shell NRs prepared on transparent fused silica plates were measured in the UV-near IR range using a Shimutsu UV3101PC Photo-Spectrometer (Nakagyo, Kyoto, Japan).
Results and discussion
Besides the ZnO (002) peak, the XRD pattern of sample B shows one broad peak located at 2θ = 26.86°. This peak is attributed to the (111) diffraction of face-centered cubic (FCC) zinc blende ZnSe (JCPDS: 37–1463). The broadening of the diffraction peak indicates the small crystallite size of the deposited ZnSe. Moreover, the ZnO (002) peak exhibits a small shift (approximately 0.2°) toward the smaller angle side, suggesting that the lattice of the ZnO cores suffers a tensile strain. This can be attributed to the growth of the ZnSe shells outside the ZnO cores since ZnSe has a much larger lattice constant than ZnO . For sample D obtained by annealing sample B at 500°C in N2, both the ZnSe (111) and the ZnO (002) peaks show an increased intensity and a narrowed FWHM compared with sample B, indicating an improvement in crystal quality of ZnSe and ZnO due to annealing. Furthermore, two additional peaks are observed at approximately 45.3° and 53.5°, respectively, which can be assigned to the (220) and (311) diffractions of cubic zinc blende ZnSe. The lattice constant of ZnSe is determined to be a = 0.568 nm. Contrast to sample B, more diffraction peaks are observed for sample C with the ZnSe (111) diffraction exhibiting a higher intensity and a narrower FWHM, indicating that sample C has a better crystallinity than sample B. The above XRD results suggest that better crystallinity of ZnO cores and ZnSe shells could be obtained either by RT deposition of ZnSe followed by post-deposition annealing or merely by depositing ZnSe at elevated temperatures.
The above optical characterization based on the measurements of transmission spectra and PL spectra reveal that the fabricated ZnO/ZnSe core/shell NRs have a photoresponse much broader than those of the constituting materials ZnO and ZnSe. The extending of photoresponse makes the ZnO/ZnSe core/shell NRs promising as absorbent materials of solar radiation in solar devices.
In this work, we studied the optical properties of vertically aligned ZnO/ZnSe core/shell NRs after morphology and structure characterization. By pulsed laser deposition of ZnSe on the surfaces of hydrothermally grown ZnO NRs, type-II ZnO/ZnSe heterojunctions constructed of ZnO cores and ZnSe shells were fabricated. The ZnO core NRs grown vertically on the substrates are composed of nanocrystallites with wurtzite structure, while the ZnSe shells, also composed of nanocrystallites, are zinc blende in crystal structure. The structures of both the ZnO cores and the ZnSe shells can be improved by post-fabrication annealing in N2. High-temperature deposition of ZnSe has also annealing effects on the structure of the ZnO cores. At room temperature, the ZnO NRs exhibit a good behavior on UV NBE emission with a weak defect-related visible emission, whereas only a weak PL is observed from the ZnO/ZnSe core/shell NRs because of the suppression of the emission from ZnO cores by the ZnSe shells. The ZnO/ZnSe core/shell NRs fabricated by depositing ZnSe at elevated temperatures are superior to the samples fabricated by depositing ZnSe at room temperature both in structure and optical properties. Multi-band luminescence including the UV NBE emission of ZnO and the blue NBE emission of ZnSe is observed from the samples fabricated by depositing ZnSe at 500°C on the hydrothermally grown ZnO NRs. In addition, the ZnO/ZnSe core/shell NRs fabricated with the deposition of ZnSe at 500°C show an extended photoresponse much broader than those of the constituting ZnO and ZnSe.
This work is supported by the National Basic Research Program of China (Contract No. 2012CB934303) and the National Natural Science Foundation of China (Contract No. 11275051). Acknowledgment is also given to the Doctoral Fund of Ministry of Education of China (Contract No. 20110071110020).
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