Axially-Resolved Luminescence Properties of Individual ZnSe

Article pubs.acs.org/JPCC

Axially-Resolved Luminescence Properties of Individual ZnSe Nanowires Junping Zhuang, Yao Liang, X. D. Xiao, and S. K. Hark* Department of Physics, The Chinese University of Hong Kong, Hong Kong, China ABSTRACT: We investigated axially resolved near-band-edge (NBE) and deep-level (DL) photoluminescence of individual ZnSe nanowires, using the techniques of twophoton excited luminescence imaging and time-correlated single photon counting, in a laser scanning confocal microscope. While the images of nanowires formed from the NBE luminescence appears to be uniform, a bright tip and a dim tail are observed in the images formed from the DL luminescence. At all locations of the nanowires, the luminescence decays are found to be dominated by a fast process at early time, followed by a slow component later. In addition, we report a flattened “U” shape distribution of the lifetimes along the length of nanowire for the NBE emission, and an elongated “L” shape distribution for the DL emission. Possible explanations for these results, involving carrier recombination dynamics and possibly plasmonic enhancements, are discussed.

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molecules within 100 nm above a metalized surface.10 When appropriate, our studies are also supported by spatially resolved spectra.

INTRODUCTION Semiconductor nanowires have attracted extensive attention due to their potential applications in nanodevices. Significant progress has already been achieved in using them for light emitters, photodetectors, and ultrasensitive biological and chemical sensors.1−3 For developing technologically viable devices, higher quality, more uniform nanowires are required. However, nanowires grown by current techniques are not always uniform in their properties along their length.4,5 Worse still is the fact that many of these properties often vary from wire to wire.6 Thus, it is important to develop a fast, noninvasive technique to characterize the uniformity of the nanowires. So far, the most common noninvasive techniques that have been used to study spatially resolved optical properties of individual nanowires are microphotoluminescence4 and cathodoluminescence mapping.7,8 However, these techniques are usually used to observe the distribution of luminescence intensities, which reveals only an aspect of the carrier recombination processes occurring in the nanowire. Other important aspects of the recombination dynamics, such as lifetimes, are not directly probed. Recently, time-resolved photoluminescence (PL) has been intensively applied to investigate the recombination dynamics of individual nanowires as a whole.4,6,9 However, we are unaware of any report on the measurement of the distribution of luminescence lifetimes over individual nanowires, except the one by Rakovich et al.,5 in which an irregular CdTe network was investigated at rather low spatial resolution. In this paper, we report studies of luminescence intensity and lifetimes of individual ZnSe nanowires along their length at the ultimate spatial resolution afforded by an optical system, using a laser scanning confocal microscope equipped with a time-correlated single photon counting (TCSPC) module. An essentially similar setup has been applied to measure the absolute positions of fluorescent © 2012 American Chemical Society

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EXPERIMENTS ZnSe nanowires were synthesized via the self-catalyzed vapor− liquid−solid growth mode on GaAs substrates in a metal− organic chemical vapor deposition reactor. The morphology and structure of the as-grown nanowires were characterized by electron microscopy in a field emission scanning electron microscope (SEM, FEI QF400) and in a high resolution transmission electron microscope (HRTEM, FEI Tecnai-20). Their optical properties were characterized by PL and cathodoluminescence spectroscopy. To study the optical properties of the ZnSe nanowires individually, they were broken off the substrate and dispersed on a cover glass that forms the bottom of a container. The nanowires were two-photon excited point-by-point with a Ti: sapphire pulse laser (Spectra-Physics) tuned to 800 nm (∼100 fs pulse width, ∼80 MHz repetition rate, average power ∼2.5 mW) using a Leica TCS SP5-II laser scanning confocal microscope. The same large numerical aperture (NA = 1.20) water immersion 63 × objective (Leica, HCX PL APO CS 63.0× 1.20 water UV) was used to focus the laser onto and to collect light emissions from the nanowires. With a proper setup of the optical system, luminescence within the spectral bands of the near-band-edge (NBE) emission, from 446 to 486 nm, and the deep-level (DL) emission, from 535 to 585 nm, were detected simultaneously by two avalanche photodiodes (Micro Received: January 3, 2012 Revised: March 23, 2012 Published: March 29, 2012 8819

dx.doi.org/10.1021/jp300051s | J. Phys. Chem. C 2012, 116, 8819−8823

The Journal of Physical Chemistry C

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Photon Devices, PDM series) in a single-photon counting mode. The luminescence time decay curves were measured using the TCSPC module (PicoQuant, PicoHarp 300), which has an overall system response time of 100 ps. The measured image size was set at 256 × 256 pixels. Considering the optical resolution (∼320 nm) of our optical system and the data fluctuations, binning of 8 × 8 pixels into a single image spot (352 × 352 nm2 physical size) was carried out whenever image data were analyzed and presented. The spectra of individual nanowires were obtained by dispersing the luminescence collected by the objective with a prism spectrometer and detecting it by a photomultiplier tube (PMT, Hamamatsu R9624). To obtain good signal-to-noise ratio in a reasonable amount of time, we had to sacrifice spectral and spatial resolution somewhat. In addition, photoand cathodo-luminescence spectra of higher resolution, measured in spectrometers different from that described above, were also obtained. These were obtained from the same batch of nanowires synthesized, but not necessarily the same individual nanowires.

Figure 2. 300 K PL spectrum with CW laser excitation and micro-PL spectra with two-photon excitation at the tip and middle of a single nanowire.

same excitation from the tip and the middle of the same nanowire are still different, especially in terms of the intensity ratios of the peaks. We think the first reflects more the power difference of pulsed and CW excitations. The increased likelihood of many body effects and partial saturations of lower energy states under intense pulsed laser excitation will be further discussed below. The second results from a lack of statistical fluctuations in single nanowires, as well as from saturation effects that suppress the low relative to the high energy luminescence. In an assembly of nanowires, each emitting a DL band slightly shifted from that of the other, the DL band tends to become statistically broadened. More importantly, the last difference provides the first indication that the optical property of a nanowire is far from uniform. Figure 3a,b shows the luminescence images formed from the NBE and DL emissions of several ZnSe nanowires, which were

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RESULTS AND DISCUSSION Figure 1a shows an SEM image of a typical ZnSe nanowire transferred onto a Si substrate. A Ga-particle is always observed

Figure 1. (a) SEM image of a typical individual ZnSe nanowire; (b) HRTEM image taken from the tip of another typical nanowire.

on the tip of the as-grown nanowires. The diameter and length of the nanowire are about 200 nm and 7 μm, respectively. In the HRTEM investigations (Figure 1b), a thin amorphous layer covering the nanowires and stacking faults, except for a narrow (tens of nanometers) region, were observed within. The stacking fault free region was always observed near the catalyst. An example of such a region is shown between the two dashed lines in Figure 1b. More detailed information about the growth and luminescence of our ZnSe nanowires are described in a previous report.11 Figure 2 compares the room-temperature PL spectra obtained from different spots on an individual nanowire using two-photon excitation with that obtained from an assembly of nanowires using 420 nm continuous wave (CW) laser excitation. A narrow near-band-gap NBE (450−470 nm) and a broad DL emission band (520−750 nm) are typically found in the spectra. However, it is noted there are considerable differences between the spectra obtained from a single nanowire and that from an assembly. First, the intensity ratios of the NBE to DL band are significantly higher in the single nanowire spectra than the spectrum of the assembly. Second, the DL emission in the former covers a much narrower spectral region than that in the latter. Third, spectra obtained by the

Figure 3. Two-photon excited luminescence images of ZnSe nanowires formed from the (a) NBE and (b) DL emissions.

well dispersed on the cover glass. The spatial distribution of the intensity of the NBE emission seems to be uniform along the nanowires, indicating that their reasonably good quality. That of the DL emission, however, is far from uniform. The short bright tip and long dim tail are suggestive of a spatial variation in the concentration of the radiative DL defects. Due to their low spatial resolution (compared to that of SEM images), it is hard to identify from these images whether the bright tip is associated with the Ga-particle or not. This query was answered by cathodoluminescence mapping in an SEM, which revealed that a Ga-particle is indeed always found attached to the bright tip. In Figure 4a,b, we show the axially resolved two-dimensional (2D) luminescence intensity decay diagrams of a single 8820



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The decay curves of the NBE and DL emissions obtained near the tip, middle, and base of the nanowire, locations indicated by the arrows in Figure 4, are shown in Figure 5a,b, respectively. The curves were well-fitted by a doubleexponential decay function: I(t ′) = A 0 + A1e−t ′ / τ1 + A 2 e−t ′ / τ2 ,

(τ1 < τ2)

(1)

where t′ = t − t0, with t being the time in the decay curve and t0 = 2 ns being the starting point of the selected data range for fitting. The amplitudes, A1, A2, and A0, and the time constants, τ1 and τ2, are the fitting parameters. Single and stretched exponential functions were also tried, and they did not fit the decays as well as the double exponential function, suggesting that there are indeed two lifetimes, particularly, for the DL emission. A characteristic lifetime ⟨τ⟩, defined by

⟨τ⟩ =

A1τ12 + A 2 τ 22 A1τ1 + A 2 τ2

(2)

describes the duration when carriers remain at the excited states after the excitation. The fitted curves are shown in Figure 5 as dashed lines, along with the lifetimes at each location. From the decay curves shown in Figure 5, it is found that the backgrounds of the decay curves of the DL emission at the tip of the nanowire are about one order higher than those in other cases, which are basically of the same order. The low backgrounds were essentially produced by measurement noises, such as the dark counts of the detectors. The high background indicates that there exist many long-lived radiative DL defects near the tip of the nanowire. For the luminescence lifetime measurements with the TCSPC technique, an important point to keep in mind is that peak carrier densities excited by the ultrashort laser pulse can be over 104 times that by a CW laser at the same average power. For instance, in our experiments, the excited carrier density was estimated to be about 3 × 1019 cm−3, with the twophoton absorption coefficient of ZnSe at 800 nm being β ∼4.0 cm/GW.12,13 At such a density, greater than the Mott density of 3 × 1018 cm−3 (estimated from an exciton Bohr radius a0 = 45 Å14 in ZnSe), the excitons are so close together that their wave functions begin to overlap.15 Thus, biexcitons or many body effects should be considered in our experiments. In this case, the recombination rate 1/τ for the carriers in the NBE states could be written as16,17

Figure 4. Axially resolved 2D luminescence intensity decay diagrams (right, pseudocolor images) of an individual ZnSe nanowire for the (a) NBE and (b) DL emissions. Also shown (left, gray scale images) are their corresponding luminescence images, which are cut out from the initial 256 × 256 piexls. The grid lines represent the pixel binning.

nanowire, together with its corresponding luminescence images. We note that, for the NBE emission (Figure 4a), the decay processes are spatially more uniform; they are just slightly slower at the ends than in the midsection of the nanowire, judging from the distance between the contour curves of 1 k and 100 counts. In addition, it is found, by noting the 10 k contour curve, that the initial intensities near the ends are slightly weaker than in the midsection. The DL emission (Figure 4b) near the bright tip, where long decay tails are observed, exhibits a rather different decay behavior from that at other locations. The initial intensities of the DL emissions at different locations are found to be about the same. It is interesting to note that the stronger DL emission is accompanied by a longer decay tail, while the opposite case is true for the NBE emission. The connections between the intensity and lifetime distributions could be understood from the different recombination dynamics involved in the NBE and DL emissions, as we shall show later.

1 = C1 + C2n + C3n2 τ

(3)

Figure 5. Normalized decay curves obtained near the bright-tip, middle, and base spots, which are indicated in Figure 4 with arrows, of the same nanowire: (a) NBE and (b) DL emissions. The dashed lines show the results of double-exponential fitting. All of the curves were normalized at the maxima. 8821



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where n is the carrier density. The three items on the righthand side of eq 3 correspond to the Shockley−Read−Hall transition, radiative transition, and Auger scattering, respectively. Noting that the carrier density n decays with time, it is not unreasonable that the decays of the NBE emission are multiple exponential in nature, of which the double exponential is but the lowest order that yields a good fit. The decays of DL emission are similar to those of the NBE emission: namely, a fast decay at early times followed by a slow component. However, the overall characteristic lifetimes tend to be longer than those of the NBE emission. Figure 6 shows the spatial variations of the characteristic lifetimes for the NBE and DL emissions, ⟨τNBE⟩ and ⟨τDL⟩,

(Zn vacancies and interstitials) of the ZnSe lattice rather than the stacking faults.18,19 These radiative centers are likely found everywhere along the nanowire, stacking fault free or not. One possible cause of the intense DL emission is an increased concentration of these centers due to segregation effects near the growth front (Ga/ZnSe interface). Another possibility is the localized surface plasmon enhanced luminescence. In the proximity of the nanosized Ga particle, light emission in resonance with a surface plasmon mode is responsible for the intense DL emission near the tip.20,21 This agrees with the florescence enhancement of chromophores often observed near metallic nanoparticles and attributed to plasmonic effects. Without further studies, it is not easy to exclude either one of the two possibilities.

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CONCLUSION In conclusion, we have investigated axially resolved luminescence intensity and lifetime of individual ZnSe nanowires. The luminescence intensity images formed from the DL emission show a short bright tip and a long dim tail, while those formed from the NBE emission are more uniform. The luminescence lifetime images show that the intensity decays are dominated by a fast decay τ1 at early time, followed by a slow one τ2, for both the NBE and DL emissions. This double exponential decay is attributed to the high density of the photoexcited carriers. A flattened “U” shape distribution was observed for the spatial variations of the characteristic NBE emission lifetime, ⟨τNBE⟩, along the length of nanowire. The longer ⟨τNBE⟩ at the ends is attributed to a higher number of surface traps in the proximity. An “L” shape distribution for the characteristic DL emission lifetime, ⟨τDL⟩, in which the long lifetimes occur near the bright tip, was observed. This distribution is possibly associated with the distribution of the radiative centers, the existence of a region free of stacking faults, and surface plasmon resonant enhancement near the tip.

Figure 6. (Above) Spatial variations of characteristic lifetimes ⟨τNBE⟩ and ⟨τDL⟩ along the length of the nanowire. (Below) Also shown are the luminescence images formed from the NBE and DL emissions of the same nanowire, with the bright tip chosen as the origin. The arrows indicate the spots where decay curves were shown in Figure 5.

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along the length of the nanowire, with the bright tip chosen as the origin. It is found that ⟨τNBE⟩ is longer at the ends than in the midsection, which makes its spatial distribution look like a flattened capital letter “U”. However, ⟨τDL⟩ is much longer near the bright tip than any other locations, hence its distribution is more like a capital letter “L” with an elongated tail. We think the additional surface states (on the truncated face) at the ends of the nanowire (compared to the midsection) should be responsible for the difference in ⟨τNBE⟩ between the ends and the midsection. The state filling of the traps in ZnSe nanowires was found to complete within a few picoseconds.18 Thus, after ∼100 ps, our system response time, more excited carriers become trapped, and this results in a lower initial free carrier density and weaker NBE luminescence near both ends (Figure 4a) than in the midsection. From eq 3, we can find that the lower free carrier density results in a longer lifetime for the NBE emission. Thus, the lifetime ⟨τNBE⟩ is longer at the ends than in the midsection. When the distributions of ⟨τDL⟩ and the luminescence intensity of DL emission in Figure 6 are examined together, it is clear that long lifetimes and intense luminescence both occur near the tip. The existence of a narrow stacking fault region at the tip (Figure 1b), suggests that reduction of bulk and surface nonradiative recombination processes is likely one of the reasons that give rise to the long lifetime.15 The DL emission of ZnSe was attributed to recombinations at intrinsic point defects

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work described in this paper was partially supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No. 417507).

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Axially-Resolved Luminescence Properties of Individual ZnSe

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