Multi-band luminescent ZnO/ZnSe core/shell nanorods and their temperature-dependent photoluminescence

Abstract

We report on the multi-band emission from one-dimensional heterostructured ZnO/ZnSe core/shell nanorods (NRs) and its temperature dependence. Aligned ZnO/ZnSe core/shell NRs constructed of wurtzite ZnO cores and zinc blende ZnSe shells were fabricated by hydrothermal growth of ZnO NRs and pulsed laser deposition of ZnSe coatings on the grown ZnO NRs. The presence of the ZnSe shells outside the ZnO NR cores strongly suppresses the radiative recombination of photogenerated electrons and holes in ZnO and consequently quenches the photoluminescence from ZnO NRs. The suppression of the radiative recombination associated with different transitions in ZnO behaves very differently at different temperatures. At room temperature, the ZnSe-coated ZnO NRs are capable of emitting a multi-band luminescence including a UV emission centered at ∼378 nm, a blue emission centered at ∼462 nm and a broad band emission ranging from 500 to 720 nm. The spectral feature of the multi-band luminescence is strongly dependent on temperature, suggesting the possibility of tuning the emitted light by simply varying the temperature and an approach to broad-band or even full-color emission from the multi-band fluorescent ZnO/ZnSe core/shell NRs.

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Introduction

There has been a great interest in broad-band light emitting semiconductors. In particular, there is a strong desire for semiconductors used for efficient full-color light emitting and display devices. With a wide direct band gap of 3.37 eV and a large exciton binding energy of 60 meV at room temperature, zinc oxide (ZnO) is recognized as one of the most prospective materials for optoelectronic applications, and efficient light emission from ZnO can be expected at room temperature.^1–3 Due to its wide band gap, however, ZnO itself can only be used in devices working in the ultraviolet (UV) region. One approach to get ZnO-based materials working in the visible region is to functionalize the surface of ZnO with a narrower band gap semiconductor used as the sensitizing material.^4–6 In this respect, nanostructured ZnO is superior to bulk ZnO in its large surface-to-volume ratio, since surface modification including surface coating has been proved to be one of the most feasible methods to construct novel nanostructures with tailored properties. Nano-heterostructures constructed of ZnO and another semiconductor, for example, one of other Zn-based II–VI semiconductors, usually exhibit improved individual and combined properties. Zinc selenide (ZnSe) is another important Zn-based II–VI semiconductor which has also been considered as a promising material for optoelectronic applications.^7–9 The structure and the band gap of ZnSe make it an ideal candidate to modify the surface of nanostructured ZnO. A ZnO/ZnSe core/shell nano-heterostructure can be constructed by covering nanostructured ZnO with a ZnSe coating. Using aligned ZnO nanorods (NRs) as cores and ZnSe thin coatings as shells, in particular, aligned one-dimensional heterostructured ZnO/ZnSe core/shell NRs show properties very distinct from those of the single constituting materials ZnO and ZnSe.^5,10–12 In particular, excellent optical properties can be obtained. Compared with ZnO and ZnSe, ZnO/ZnSe core/shell NRs show increased optical absorption and extended absorption region, hence exhibit improved photo-response in a wide spectral region from UV to near infrared (near-IR),^11–13 which facilitates the generation of electron–hole pairs by optical excitation. In the heterostructured ZnO/ZnSe core/shell NRs, in addition, the type II band alignment is favorable for charger transfer and separation, which suppresses the radiative recombination of photogenerated electrons and holes. The efficient photogeneration of electrons and holes and the suppressed recombination of photogenerated electrons and holes make ZnO/ZnSe core/shell NRs promising for enhancing light harvesting efficiencies in photoelectronic applications.

Our preliminary studies have revealed that ZnO/ZnSe core/shell NRs not only exhibit improved photo-response in a wide region from UV to near-IR, but also can emit a multi-band luminescence at room temperature including a UV band, a blue band and a broad green-to-red band.¹³ In the present work, we report more details on the luminescence features of the multi-band luminescent ZnO/ZnSe core/shell NRs including the temperature dependence of the luminescence. By comparing the temperature-dependent photoluminescence of ZnO NRs and ZnO/ZnSe core/shell NRs, the mechanisms responsible for the modified optical properties including the suppressed luminescence from ZnO cores and the multi-band luminescence of ZnO/ZnSe core/shell NRs were discussed.

Results and discussion

Fig. 1 illustrates typical field-emission scanning electron microscope (FESEM) images of the bare ZnO NRs and the ZnSe-coated ZnO NRs fabricated on Si (100) substrates before and after annealing. It can be seen that the ZnO NRs have grown nearly with their axes perpendicular to the substrate and are generally shaped with hexagonal prisms with smooth surfaces (Fig. 1a). The bare ZnO NRs have an average diameter of approximately 90 nm and a length of about 1.5 μm. After the deposition of ZnSe, the ZnO rods are fully covered by the ZnSe coatings, exhibiting slightly increased rod diameters and rough surfaces, as illustrated in Fig. 1b. Post-fabrication annealing results in an improvement in both morphology and structure. As shown in Fig. 1c, the surfaces of the ZnSe coatings become smoother, and the ZnSe shells get thinner and cover the ZnO nanorods more tightly after annealing. The samples fabricated on quartz exhibit similar morphology.

Fig. 1 FESEM images of (a) annealed bare ZnO NRs, (b) as-fabricated ZnO/ZnSe core/shell NRs and (c) annealed ZnO/ZnSe core/shell NRs.

Structure characterization by X-ray diffraction (XRD) reveals that the grown ZnO NRs are structured with hexagonal wurtzite with the preference of (002) orientation along the c-axis, while the deposited ZnSe coatings exhibit cubic zinc blende structure with the preference of (111) orientation. The fabricated core/shell nano-heterostructures are therefore composed of wurtzite ZnO nanorod cores and zinc blende ZnSe coating shells. The crystal structures of both the ZnO cores and the ZnSe shells are improved after annealing in N₂. Further details of the XRD characterization can be found in the ESI S1 and Fig. S1.† The wurtzite structure of the ZnO cores and the zinc blende structure of the ZnSe shells as well as the improvement in crystallinity due to post-fabrication annealing are confirmed through vibrational mode analysis and phase identification based on Fourier-transform infrared (FTIR) spectroscopy (see ESI S2 and Fig. S2† for a detailed description of the FTIR measurements and analysis). Raman backscattering measurements provide another evidence for the wurtzite structured ZnO cores and the zinc blende structured ZnSe shells as well as for the effects of post-fabrication annealing on the crystal structure. A more detailed analysis of the results obtained by Raman backscattering measurements is provided in ESI S3 and Fig. S3.†

The heterostructure of the annealed ZnO/ZnSe core/shell NRs is further confirmed by TEM characterization. As shown in Fig. 2a, the diameter of ZnO nanorod is about 90 nm and is fully covered by ZnSe shell which is about 20 nm in thickness. Fig. 2b is the high resolution TEM image of the interface region between the ZnO core and the ZnSe shell, revealing that the ZnO core and the ZnSe shell have a clear boundary, though the interface between ZnO and ZnSe seems not perfect. The lattice fringe spacing of 0.52 nm matches well with the interplanar distance of the (0001) lattice planes of wurtzite ZnO, while that of 0.33 nm corresponds to the (111) planes of zinc blende ZnSe. Near the interface, in addition, there is a transition layer with a fringe spacing of 0.57 nm corresponding to the (011) planes of zinc blende ZnSe. The selected area electron diffraction (SAED) pattern shown in Fig. 2c indicates a bright diffraction pattern corresponding to the ZnO (002) planes, whereas the diffraction pattern of ZnSe cannot be seen clearly.

Fig. 2 (a) TEM image of a single annealed ZnO/CdS nanorod, (b) HRTEM image and (c) SAED pattern of the interface region between ZnO core and ZnSe shell.

The annealed bare ZnO NRs show weak optical absorption in the visible and near-IR spectral region with an absorption edge at about 385 nm. The absorption increases abruptly near the absorption edge, as shown in Fig. 3 which displays the absorption spectra of the samples fabricated on quartz substrates. The abrupt increase in the absorption near 385 nm is attributed to the absorption across the band gap of wurtzite ZnO.^1,2 The as-fabricated ZnO/ZnSe NRs exhibit increased optical absorption in the whole spectral region, and additional absorption is found to extend to the region below the ZnSe band gap into the near-IR region. After annealing, the optical absorption of the ZnSe-coated ZnO NRs decreases significantly, but is still much stronger than that of the bare ZnO NRs. The decrease in the optical absorption of the annealed ZnO/ZnSe NRs as compared to that of the sample before annealing is due to the improvement in the structure of the ZnO cores and the ZnSe shells as well as in the interface between ZnO and ZnSe. In addition to the absorption edge corresponding to the band gap of wurtzite ZnO, the annealed ZnO/ZnSe core/shell NRs show a second absorption edge near 465 nm. This absorption edge is corresponding to the band gap of zinc blende ZnSe and can be attributed to the trans-band absorption of ZnSe.⁸ The improvement in the structure of the ZnSe coatings makes this absorption edge well-exhibited as compared to that of the as-fabricated sample. Moreover, the annealed ZnO/ZnSe core/shell NRs have an additional broad absorption band in the spectral region ranging from 650 to near-IR, which should result from the transitions across the so-called effective band gap formed between the conduction band minimum of ZnO and the valence band maximum of ZnSe of the constructed ZnO/ZnSe heterostructured,¹² as well as from the interfacial transitions occurring in the heterostructured sample. It is apparent that attributed to the type II band alignment in the heterostructures constructed of ZnO and ZnSe, the ZnSe-coated ZnO NRs exhibit an improved photo-response including increased optical absorption and extended absorption region as compared to that of the bare ZnO NRs. Therefore, the fabricated ZnO/ZnSe core/shell NRs are expected to be a promising ZnO-based optoelectronic material which can be used in devices working in the UV and visible region and even in the near-IR region due to its high optical absorbance and broad absorption region.

Fig. 3 Optical absorption spectra of annealed bare ZnO NRs (1), as-fabricated ZnO/ZnSe core/shell NRs (2) and annealed ZnO/ZnSe core/shell NRs (3) fabricated on quartz.

Fig. 4a shows the photoluminescence (PL) spectra of the samples measured at room temperature (300 K). With 325 nm light excitation, the annealed bare ZnO NRs emit a luminescence including a strong and narrow UV band peaking at ∼378 nm with a full width at half maximum (FWHM) of about 15 nm and a weak visible band maximizing at ∼540 nm and spreading over a wide spectral region. The UV emission is attributed to the near band edge (NBE) emission of ZnO,^2,14,15 and the visible emission is associated with the defects in ZnO such as oxygen vacancies.^16,17 The weak visible luminescence suggests the low concentration of defects in the annealed bare ZnO NRs. The UV NBE emission of the annealed ZnO NRs is nearly twice as that of the as-grown ZnO NRs (not shown here) because of the improvement in the crystallinity of ZnO after annealing.

Fig. 4 PL spectra of annealed bare ZnO NRs (1), as-fabricated ZnO/ZnSe core/shell NRs (2) and annealed ZnO/ZnSe core/shell NRs (3) recorded at (a) 300 K and (b) 7 K. The inset in (a) shows the multi-band PL spectrum of annealed ZnO/ZnSe core/shell NRs recorded at 300 K, and the inset in (b) compares the ZnO NBE emissions of annealed bare ZnO NRs, as-fabricated ZnO/ZnSe core/shell NRs and annealed ZnO/ZnSe core/shell NRs recorded at 7 K.

The PL from the ZnO NRs is remarkably reduced after the coverage of the ZnO NRs by the ZnSe coatings (spectrum 2 in Fig. 4a). In particular, the UV ZnO NBE emission is reduced by a factor over 50 in intensity. One of the main contributions to the reduction in the measured luminescence from ZnO is the suppression of radiative recombination of photogenerated electrons and holes in ZnO, since in the type II hetero-nanostructure composed of ZnO core and its outside ZnSe shell, the staggered energy band alignment is favorable for spatially separating electrons and holes, which effectively suppresses the radiative recombination of photogenerated electrons and holes and hence quenches the luminescence.^12,13 Fig. 5 shows schematically the energy band alignment of ZnO/ZnSe heterostructures and the transfer and spatial separation of photogenerated electrons and holes. The reduction in the measured ZnO luminescence of the ZnSe-coated ZnO NRs due to the suppression of radiative recombination in ZnO will be supported by the temperature-dependent NBE emission of ZnO as described below. The absorption of the incident photons in the exciting 325 nm laser beam and the emitted photons in the emitted luminescence from the ZnO cores by the ZnSe shells might also contribute to the reduction in the measured luminescence. The interfacial trapping or defect states can also result in reducing photoluminescence. After annealing in N₂, the UV luminescence of the ZnSe-coated ZnO NRs corresponding to the NBE emission of ZnO is slightly enhanced, but still much weaker than that of the annealed bare ZnO NRs. The enhancement in the ZnO NBE emission is attributed to the improvement in the structure of the ZnO cores after annealing. Since the optical absorption of the annealed ZnO/ZnSe NRs is lower than that of the as-fabricated ZnO/ZnSe NRs, the absorption of the exciting 325 nm light and the emitted luminescence from the ZnO cores by the ZnSe shells should not be the main factors reducing the measured PL from the ZnSe-coated ZnO NRs. N₂ annealing also improved the interface quality, which may also contribute to enhance photoluminescence. More likely, however, the improvement in the interfacial quality facilitates charges to transfer across the interface between the ZnO NRs and the ZnSe coatings, promoting spatial separation of photogenerated electrons and holes. The reduction of ZnO NBE emission because of the promoted separation of electrons and holes due to the improved quality of interface counteracts in part the enhancement of NBE emission from the ZnO NRs due to the improvement of the crystal and interfacial quality.

Fig. 5 Schematic diagram showing the energy band alignment, the transfer and spatial separation of photogenerated electrons and holes, and the UV, blue and green emissions and the associated transitions.

It is worthwhile noting the multi-band emission from the annealed ZnO/ZnSe core/shell NRs, as shown in Fig. 4a (spectrum 3) and its inset. In addition to the UV band attributed to the ZnO NBE emission, the luminescence is composed of a narrow blue band centered at ∼462 nm and a broad dominant emission ranging from 500 to 720 nm. The blue band is emitted from the ZnSe shells and associated with the NBE emission of ZnSe.^7,18 The broad emission seems to consist of three components centered at ∼530, 615 and 645 nm, respectively. The former two emission components are attributed to the defect-related emissions and associated with defects such as vacancies in ZnO and ZnSe, respectively.^7,8,16,17,19 The red emission centered at about 645 nm is attributable to the radiative recombination of the electrons in the conduction band minimum of ZnO with the holes in the valence band maximum of ZnSe, i.e. the radiative transitions across the effective band gap of heterostructured ZnO/ZnSe formed between the conduction band minimum of ZnO and the valence band maximum of ZnSe.¹² This red emission and its associated radiative transitions across the effective band gap are schematically shown in Fig. 5 together with the transitions associated with the UV ZnO NBE emission and the blue ZnSe NBS emission. As will be described below, additionally, the multi-band emission of the annealed ZnO/ZnSe core/shell NRs presents a strong dependence on temperature, which provide a further support that the reduction in the measured ZnO luminescence of the ZnSe-coated ZnO NRs is mainly attributed to the suppression of radiative recombination of photo-generated electrons and holes in ZnO in the heterostructured ZnO/ZnSe core/shell NRs.

At a reduced temperature of 7 K, the UV ZnO NBE emission of the annealed bare ZnO NRs and that of the annealed ZnO/ZnSe core/shell NRs increase significantly with a remarkable blue shift and a much narrower bandwidth as a result of the freezing of phonons and the quenching of non-radiative recombination processes at low temperatures,^14,20 as shown in Fig. 4b. In addition to the increased intensity and the narrowed bandwidth of the UV ZnO NBE emission, the luminescence of the annealed ZnO/ZnSe core/shell NRs exhibits a very different spectral feature. The blue ZnSe NBE emission and the green emission from the transition associated with the defect-related deep levels in ZnO increase remarkably, while the orange defect-related emission from ZnSe and the red emission ascribed to the radiative transitions across the effective band gap remain weak. The inset in Fig. 4b compares the UV ZnO NBE emission of the samples recorded at 7 K.

One can see that at 7 K, the ZnO NBE emission of the annealed ZnO/ZnSe core/shell NRs has a peak intensity nearly equal to that of the annealed bare ZnO NRs, but with a much narrower bandwidth. The ZnO NBE emission of the bare ZnO NRs has an FWHM of about 25 meV and includes the emissions arising from the radiative recombination processes associated with free excitons, neutral donor-bound excitons and neutral acceptor-bound excitons as well as donor-to-acceptor pair transitions.^14,15 They are labelled by FX, D⁰X, A⁰X and DAP, respectively, in Fig. 6a which illustrates the high-resolution ZnO NBE emission of the bare ZnO and its Gaussian deconvolution. In contrast, the FWHM of the ZnO NBE emission of the ZnSe-covered ZnO NRs is very small (∼5 meV) and dominated by the emissions attributed to the bound exciton-associated transition at neutral donors and to the radiative recombination processes associated with free excitons, as shown in Fig. 6b, revealing the suppression of the other radiative recombination processes in the ZnO NRs covered by the ZnSe coatings. For the annealed ZnO/ZnSe core/shell NRs, moreover, the D⁰X peak of the ZnO NBE emission presents a small red shift (∼4 meV) as compared to that of the annealed bare ZnO NRs.

Fig. 6 (a) High-resolution ZnO NBE emission of annealed bare ZnO NRs recorded at 7 K and its Gaussian deconvolution. (b) Comparison of the high-resolution ZnO NBE emissions of annealed bare ZnO NRs (1), as-fabricated ZnO/ZnSe core/shell NRs (2) and annealed ZnO/ZnSe core/shell NRs (3) recorded at 7 K.

Fig. 7a and b illustrate the temperature evolution of the UV ZnO NBE emissions of the annealed bare ZnO NRs and the annealed ZnO/ZnSe core/shell NRs, respectively. Unlike the ZnO NBE emission of the bare ZnO NRs, the ZnO NBE emission of the ZnO/ZnSe core/shell NRs has a much narrower bandwidth at low temperatures. The emissions arising from the radiative recombination processes associated with neutral acceptor-bound excitons and donor-to-acceptor pairs in ZnO are quenched more remarkably than those associated with neutral donor-bound excitons and free excitons for the annealed ZnO/ZnSe core/shell nano-heterostructures. Very different from the luminescence of the bare ZnO NRs, the emissions arising from the radiative transitions associated with neutral acceptor-bound excitons and donor-to-acceptor pairs are strongly quenched in the ZnO/ZnSe core/shell NRs as compared to these two emissions of the bare ZnO NRs, while those due to the other two radiative recombination processes are less quenched, though they are much weaker than their counterparts of the bare ZnO NRs at temperatures of 35 K and above. At temperatures lower than 20 K, the latter two emissions are almost unaffected by the ZnSe coatings as compared to those of the bare ZnO NRs, especially the one due to the radiative recombination associated with neutral donor-bound excitons. The quenching of the luminescence becomes more drastic as the temperature increases and at temperatures above 50 K, all luminescence components of the UV ZnO NBE emission are strongly quenched for the annealed ZnO/ZnSe core/shell NRs. Based on the comparison of the temperature-dependent ZnO NBE emission of the ZnSe-covered ZnO NRs with that of the bare ZnO NRs, it can be concluded that the reduction of the measured room-temperature photoluminescence of the ZnO/ZnSe core/shell NRs is mainly attributed to the suppression of the radiative recombination of photogenerated electrons and holes in the sample, rather than the absorption of the exciting light of the 325 nm laser beam and the emitted luminescence from the ZnO cores by the ZnSe shells. At least, this should be the case for the ZnO NBE emission of the annealed ZnO/ZnSe core/shell NRs. Furthermore, one can also deduce that in the ZnSe-covered ZnO NRs, the suppression of the radiative recombination processes associated with neutral acceptor-bound excitons and donor-to-acceptor pairs occurs significantly at various temperatures, while the radiative recombination processes associated with neutral donor-bound excitons and free excitons are less suppressed at temperatures below 35 K.

Fig. 7 Temperature dependent UV ZnO NBE emission spectra of (a) annealed bare ZnO NRs and (b) annealed ZnO/ZnSe core/shell NRs recorded at temperatures ranging from 10 to 300 K.

Fig. 8 compares the peak intensity and the peak wavelength of the ZnO NBE emission of the annealed ZnO/CdS core/shell NRs with those of the annealed bare ZnO NRs at temperatures ranging from 10 K to 300 K. For both the bare ZnO NRs and the ZnO/ZnSe core/shell NRs, the peak wavelength of the ZnO NBE emission red shifts continuously as the temperature increases. These two kinds of NRs emit strong UV ZnO NBE emission at 10 K and the peak intensity of the ZnO NBE emission decreases as the temperature increases. However, the ZnO NBE emission of the annealed ZnO/ZnSe core/shell NRs decreases more rapidly than that of the annealed bare ZnO NRs. At 50 K, the peak intensity of the ZnO NBE emission of the annealed ZnO/ZnSe core/shell NRs decreases to less than 4% of that at 10 K. For the annealed bare ZnO NRs, in contrast, the peak intensity of the ZnO NBE emission at 50 K is about 27% of that at 10 K.

Fig. 8 Peak intensities and peak wavelengths of UV ZnO NBE emissions of annealed bare ZnO NRs and annealed ZnO/ZnSe core/shell NRs as functions of temperature.

Fig. 9 illustrates the temperature-dependent multi-band photoluminescence of the annealed ZnO/ZnSe core/shell NRs at temperatures ranging from 7 to 300 K. It can be seen that the luminescence feature of the sample varies strongly with temperature. In general, all the luminescence components increase in intensity with the temperature decreasing. Although still remaining at low intensity at temperatures around 200 K, the narrow bands attributed to the ZnO and ZnSe NBE emissions increase more rapidly than the broad visible luminescence consisting of the defect-related emissions and the emission attributed to the radiative transitions across the effective band gap as the temperature further deceases. At temperatures below 20 K, the UV ZnO NBE emission and the blue ZnSe NBE emission become the predominating luminescence components. In addition, both the UV ZnO NBE emission and the blue ZnSe NBE emission present a monotonic blue shift as the temperature decreases, while the three emission components composing the broad luminescence from 500 to 720 nm exhibit no obvious shift. The inset in Fig. 9 compares the PL spectra of the annealed ZnO/ZnSe core/shell NRs measured at 7 K and 300 K. The peak positions of the ZnO NBE emission and the ZnSe NBE emission blue shift from 378.2 and 462.0 nm at 300 K to 368.6 and 449.5 nm at 7 K, respectively, or from 3.279 and 2.684 eV in photon energy to 3.364 and 2.759 eV, respectively. Therefore, both the ZnO and ZnSe NBE emissions of the annealed ZnO/ZnSe core/shell NRs at temperatures below 20 K are predominantly associated with the excitons bound to neutral donors in ZnO and ZnSe, respectively, revealing that at low temperatures the transitions associated with neutral donor-bound excitons are the dominating radiative recombination channels near the band gaps of ZnO and ZnSe.

Fig. 9 Temperature dependent PL spectra of annealed ZnO/ZnSe core/shell NRs. The inset shows the PL spectra recorded at 300 K and 7 K.

Fig. 10 plots the variations of the peak wavelengths and the normalized peak intensities of the UV ZnO NBE emission and the blue ZnSe NBE emission with temperature for the annealed ZnO/ZnSe core/shell NRs which are capable of emitting multi-band luminescence. Similar with the UV ZnO NBE emission, the intensity of the blue ZnSe NBE emission decreases as the temperature increases together with a monotonic red shift. However, the intensity decrease of the ZnSe NBE emission with the increase of temperature is not as fast as that of the ZnO NBE emission. At 50 K, the intensity of the ZnSe NBE emission decreases to about 20% of that at 7 K, against to the decrease to less than 4% for the ZnO NBE emission, suggesting that the temperature required for the freezing of phonons and the quenching of non-radiative recombination processes in the ZnO cores is lower than that for the freezing of phonons and the quenching of nonradiative recombination processes in the ZnSe shells.

Fig. 10 Peak wavelengths and normalized peak intensities of UV ZnO NBE emission and blue ZnSe NBE emission of annealed ZnO/ZnSe core/shell NRs as functions of temperature.

The spectral feature of the photoluminescence from the multi-band luminescent ZnO/ZnSe core/shell NRs varies strongly with temperature. Fig. 11 depicts the normalized PL spectra of the annealed ZnO/ZnSe core/shell NRs recorded at 300, 270, 250, 200 and 50 K. At room temperature (300 K), as described above, the PL spectrum is dominated by the visible luminescence, especially by the orange defect-related emission of ZnSe and the red emission attributed to the radiative transitions across the effective band gap of heterostructured ZnO/ZnSe. Generally, the intensities of all luminescence components increase as the temperature decreases. Among the three components composing the broad luminescence ranging from 500 to 720 nm, however, the green defect-related emission of ZnO increases more rapidly than the other two components and gradually dominates the luminescence below 250 K. Upon further decrease in temperature to ∼50 K, the annealed ZnO/ZnSe core/shell NRs emit an intense blue ZnSe NBE emission in addition to the dominating green defect-related emission of ZnO. The variation in the spectral feature of the multi-band emission from the ZnO/ZnSe core/shell NRs with temperature suggests a possibility of tuning the emitted light by simply varying temperature. And it is interested to note from Fig. 11 that at temperature ∼270 K, the aligned heterostructured ZnO/ZnSe core/shell NRs emit an intense visible luminescence with a rather broad and flat spectral region from 500 to 700 nm. Moreover, the co-existence of the blue, green and red bands in the luminescence encourages a research for full-color emission from heterostructures composed of nanostructured ZnO and ZnSe.

Fig. 11 Normalized PL spectra of annealed ZnO/ZnSe core/shell NRs recorded at 300, 270, 250, 200 and 50 K. The shown spectra are moved vertically for clarity.

Experimental

ZnO/ZnSe core/shell NRs were fabricated by pulsed laser deposition of thin ZnSe coatings on hydrothermally grown ZnO NRs. Polished n-type Si (100) wafers and double-side polished quartz plates were used as substrates after being chemically cleaned to remove surface contaminants and natural oxide layer. Prior to the growth of ZnO NRs, a dense nanocrystalline ZnO (nc-ZnO) film (∼20 nm in thickness) was first deposited on a cleaned substrate by oxygen plasma assisted pulsed laser deposition using a pure metallic zinc target²¹ and annealed at 400 °C in air for 1 h. The annealed nc-ZnO layer served as a buffer layer between the grown ZnO NRs and the lattice mismatched substrate as well as a seed layer effectively assisting the nucleation of ZnO and the growth of ZnO NRs. The nc-ZnO seeded substrate was immersed into a hydrothermal precursor solution for the growth of ZnO NRs.²² The hydrothermal precursor solution was prepared by mixing 0.04 M hexamethylenetetramine (HMT) and 0.04 M zinc nitrate hexahydrate [Zn(NO₃)₂·6H₂O] into 1 L de-ionized water. The hydrothermal growth of ZnO NRs was carried out at a temperature of 90 °C in a thermostatic water bath for 6 h. All the chemicals were ≥99.0% in purity and used as purchased without further purification. Served as the cores, the grown ZnO NRs were covered with thin ZnSe coatings as the shells by pulsed laser ablating a sintered ZnSe target in vacuum (∼10⁻⁴ Pa) at room temperature for 30 min. The ZnSe target was ablated by the second harmonic of a Q-switched Nd:YAG laser (wavelength 532 nm, pulse width 5 ns, pulse repetition 10 Hz) with a laser fluence of 2 J cm⁻². The fabricated ZnO/ZnSe core/shell NRs were then annealed at 500 °C in a flowing N₂ atmosphere (∼10⁵ Pa) for 1 h. A bare ZnO NRs sample was also annealed under the same conditions as a reference sample.

The sample morphology was examined using a field-emission scanning electron microscope (FESEM, Hitachi S-4800). The sample structure was characterized by X-ray diffraction (XRD) with a Rigaku D/MAX 2550 VB/PC X-ray diffractometer using Ni-filtered Cu Kα radiation (λ = 0.15406 nm). The sample structure was also analyzed by Fourier-transform infrared (FTIR) spectroscopy through vibrational mode analysis and phase identification with a Bruker Vertex 80 V spectrometer. By exciting the samples with 325 nm laser light and recording the scattered spectra with a Jobin-Yvon LabRAM HR 800 UV micro-Raman spectrometer, Raman backscattering measurements were performed also for vibrational mode analysis and structure characterization. The microstructure of the components composing the heterostructured ZnO/ZnSe NRs were examined by transmission electron microscopy (TEM) and selected-area electron diffraction (SAED) using a Tecnai G² F20 S-Twin electron microscope.

The absorption spectra of the samples fabricated on quartz substrates were measured in the UV–near-IR spectral region using a Shimadzu UV3150PC Photo-Spectrometer. The photoluminescence (PL) of the samples was measured at room temperature (300 K) and cryogenic temperatures down to 7 K by exciting the samples normally with an un-polarized 325 nm He–Cd laser light beam. The measured samples were fixed to the cold holder of a closed-cycle refrigerator (Arscryo, DE-204) and cooled down to desired temperatures. The emitted luminescence was collected also at the normal direction, resolved by a 0.5 m spectrometer (Acton Research, Spectra Pro 500i) and recorded by an intensified charge-coupled device (ICCD) (Andor Technology, iStar DH720).

Conclusions

We have achieved in multi-band emitting in one-dimensional heterostructured ZnO/ZnSe core/shell NRs fabricated by pulsed laser deposition of ZnSe coatings on hydrothermally grown ZnO NRs. The ZnO cores are featured with hexagonal wurtzite structure and the ZnSe shells are cubic zinc blende in structure. The crystallinity of both the wurtzite ZnO cores and the zinc blende ZnSe shells can be improved by post-fabrication annealing in N₂. The presence of the ZnSe shells outside the ZnO cores greatly suppresses the radiative recombination of photogenerated electrons and holes in the nano-heterostructured ZnO/ZnSe core/shell NRs, resulting in a significant quenching of the photoluminescence from the ZnO nanorod cores. The photoluminescence quenching is strongly dependent on temperature because of the temperature-dependent suppression of the radiative transitions in the ZnSe-covered ZnO NRs. After annealing in N₂ atmosphere, the ZnO/ZnSe core/shell NRs are capable of emitting a multi-band luminescence including a UV band corresponding to the ZnO NBE emission, a blue band corresponding to the ZnSe NBE emission, and a broad band ranging from 500 to 720 nm attributed to the defect-related emissions of ZnO and ZnSe and the emission originating from the radiative transitions across the effective band gap formed in the nano-heterostructures composed of ZnO and ZnSe. The spectral features of the multi-band luminescence show a strong dependence on temperature. The temperature-dependent luminescence suggests an approach to the tuning of the emitted luminescence by varying temperature. More interesting and encouraging, it is possible to obtain a broad-band or even full-color emission from aligned one-dimensional nano-heterostructures constructed of ZnO cores and ZnSe shells.

Acknowledgements

This work is supported by the National Natural Science Foundation of China (Contract No. 11275051) and the Municipal Natural Science Foundation of Shanghai (15ZR1403300).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Detailed description of structure characterization by XRD, FTIR spectroscopy and Raman backscattering measurements. See DOI: 10.1039/c6ra21186k

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