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J. Phys. Chem. B 1999, 103, 3128-3137
Ultrafast Electronic Relaxation Dynamics in PbI2 Semiconductor Colloidal Nanoparticles: A Femtosecond Transient Absorption Study A. Sengupta,† B. Jiang,‡ K. C. Mandal,§ and J. Z. Zhang*,† Department of Chemistry, UniVersity of California, Santa Cruz, California 95064; State Key Lab of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Dalian, 116023, China; and EIC Laboratories, Inc., 111 Downey Street, Norwood, Massachusetts 02062 ReceiVed: October 27, 1998; In Final Form: February 18, 1999
We report the first direct measurements of ultrafast electronic relaxation dynamics in PbI2 colloidal nanoparticles using femtosecond transient absorption spectroscopy. The PbI2 nanoparticles were prepared using colloidal chemistry methods in different solvents, including ethanol, 2-propanol, 1-butanol, water, and acetonitrile, as well as in poly(vinyl alcohol) (PVA) matrix. The particle sizes were determined using low- and high-resolution transmission electron microscopy and atomic force microscopy, which provided direct evidence of photodegradation of the nanoparticles. The ground state electronic absorption spectra of aged PbI2 nanoparticles in acetonitrile and alcohol solvents showed two major peaks near 360 and 292 nm, which slightly blue shift with decreasing size. In aqueous solution containing PVA a new sharp excitonic peak appeared at 414 nm, indicative of nanoparticle formation. With excitation at 390 nm and probing in the visible to near-infrared region, the electronic relaxation dynamics in PbI2 nanoparticles were directly monitored. The electronic relaxation is found to be sensitive to solvent and insensitive to particle size. In acetonitrile the relaxation was dominated by a 75 ps decay. In alcohol solvents, in addition to a 75 ps decay, a fast 6 ps decay was observed. The relaxation in aqueous PVA solution featured a double exponential decay with time constants of 1 and 40 ps. There appeared to be oscillations at early times with a period changing with solvent but not with particle size. The dynamics observed were somewhat dependent on the probe wavelength and independent of the excitation intensity. The results suggest that the surface plays a major role in the electronic relaxation process of PbI2 nanoparticles. The influence of particle size is relatively minor in the size range studied (3-100 nm), probably because the relaxation is dominated by surface characteristics that do not vary significantly with size and/or the size is much larger than the exciton Bohr radius (1.9 nm) and thereby spatial confinement is not significant in affecting the relaxation process.
Introduction Semiconductor nanoparticles are interesting materials because they have chemical and physical properties different from those of the bulk and isolated atoms or molecules with the same chemical composition. Residing in the mesoscopic regime between molecular and bulk regimes, nanoparticles have unique electronic and optical properties because of their small size and extremely large surface-to-volume (S/V) ratio. Their unique properties have potential applications in a number of areas including microelectronics, nonlinear optics, photocatalysis, and photoelectrochemistry. The large percentage of surface atoms can introduce a high density of surface states. These surface states can fall within the band gap and trap charge carriers (electrons and holes), which in turn can significantly influence the charge carrier behavior and other properties of the nanoparticles. Recent studies in our lab and by others, using timeresolved techniques to directly measure the charge carrier dynamics, have elucidated how charge carriers interact with phonons, other charge carriers, and the surface.1-13 In semiconductor materials, spatial confinement of charge carriers in multilayered or multi-quantum-well structures has * Corresponding author. † University of California. ‡ Dalian Institute of Chemical Physics. § EIC Laboratories, Inc.
many potential utilities.14 Several recent reports on layered semiconductor materials like PbI2, BiI3, HgI2, Bi2S3, and Sb2S3 showed interesting properties of such materials.14,15 Bulk lead iodide, PbI2, is an important material for digital X-ray imaging and optical detector applications and has been well characterized.16,17 PbI2 nanoparticles have been synthesized and the quantum confinement effect has been studied spectroscopically.14,18-21 As an anisotropic semiconductor, PbI2 has a CdI2 type of layered structure, with repeat unit of hexagonally closed-packed layer of Pb2+ sandwiched between two layers of I- in the crystal.14,16 The cations in the middle layer are octahedrally surrounded by six anions, forming the top and bottom layers of the sandwich.22 Each anion is thus interacting with three cations within a “sandwich” as well as weakly interacting with the adjacent “sandwich”. A large number of structural polytypes of PbI2 can occur,23 depending on the method of preparation. Different polytypes show different optical properties. Absorption and reflectivity measurements on PbI2, a direct band gap semiconductor, have shown a strong excitonic peak in the 480-490 nm (2.6-2.5 eV) range. This wavelength variation is attributed to effects of sample strain on aging and to different structural polytypes.23 PbI2 also represents a special case of much heavier electron mass than hole mass (me . mh), especially in the direction parallel to the c-axis, which enhances electron localization.19 To date, the dynamic properties of
10.1021/jp9842345 CCC: $18.00 © 1999 American Chemical Society Published on Web 04/02/1999
PbI2 Semiconductor Colloidal Nanoparticles photogenerated charge carriers in PbI2 nanoparticles have not been characterized. In this paper we report the first direct study of the electronic relaxation dynamics in PbI2 nanoparticles using femtosecond transient absorption spectroscopy. In the following sections, we will first provide some experimental details of the synthesis of PbI2 colloidal nanoparticles in different solvents, including ethanol, 2-propanol, 1-butanol, acetonitrile, and aqueous solution containing PVA as well as of their characterizations using microscopy and spectroscopic techniques. We will then analyze the dynamics data and focus on electronic relaxation processes in PbI2 nanoparticles. Oscillations observed at early times with a period of 6 and 1.7 ps in acetonitrile and alcohol solvents, respectively, are briefly discussed. Also given is a detailed discussion of the photodecomposition process of PbI2 nanoparticles in different solvents. All the observations indicate that the electronic relaxation processes in PbI2 colloidal nanoparticles are sensitive to the surface but insensitive to particle size in the range studied (3-100 nm). Possible interpretations of the observations are provided. Experimental Section The PbI2 nanoparticles were prepared at room temperature without using surfactants following the procedure of ref 14. We have extended the synthesis in acetonitrile and water to include 1-butanol, 2-propanol, and ethanol. Typically, 5 mL of a 0.01 M aqueous solution of lead nitrate was added to 100 mL of the solvent of interest. This solution was then vigorously stirred as 2.5 mL of a 0.05 M aqueous solution of potassium iodide solution was rapidly injected in it by syringe. The environments of the PbI2 nanoparticles were kept anionic by having [I-] at a slightly higher concentration than that required for a [I-]:[Pb2+] ratio of 2:1 in solution. Preliminary X-ray photoelectron spectroscopy (XPS) measurements showed that the iodide-tolead ratio is 3 to 1 for PbI2 nanoparticles prepared in acetonitrile. The particle sizes have been determined by using low- and high-resolution transmission electron microscopy (TEM) as well as atomic force microscopy (AFM). The particle size of the freshly prepared samples is dependent on the solvent used, as verified by TEM. The more polar the solvent is, the more soluble PbI2 is and thus the smaller the particle size is, similar to what has been reported previously.13 We have noticed significant photodecomposition of large particles due to aging in the presence of light, resulting in smaller particles. Photodecomposition due to aging has been observed previously for PbI2 nanoparticles in silica glass.18 After a few weeks of aging under room light or a few days of illumination with 390 nm laser light, the increase in optical density stopped and the particle size reached a limit of about 2.5-3 nm, as confirmed by both high-resolution TEM and AFM. The samples seemed to be stable afterwards for many weeks. The PbI2 nanoparticles in aqueous solution containing PVA (MW 86 000-100 000) were prepared following the procedure of ref 24. Typically a saturated boiling aqueous PbI2 solution was prepared in the presence of 0.5% PVA. This solution was then cooled either quickly in ice (1 min) or slowly to room temperature. A transparent light yellow colloidal solution with small opalescence of PbI2 was obtained by fast cooling, and TEM measurements showed that particles with a large size distribution (30-100 nm) were present in the fresh solution, which caused the opalescence of colloidal solution. The slow cooling (∼10 min) gave a muddy yellow colloidal solution, indicating the formation of much larger nanoparticles, compared to the fast cooling procedure. These particles readily dissolved
J. Phys. Chem. B, Vol. 103, No. 16, 1999 3129 upon dilution and heating. The free-standing thin films of PbI2 in PVA were prepared by heating and annealing these solutions on glass microscope slides and later peeling them off. The FTIR studies were performed with these free-standing films. After aging under light, the high-resolution TEM showed that the particle size in the PVA samples also decreased and had a distribution of 4-14 nm. The UV-visible spectra were measured on a Hewlett-Packard diode array spectrophotometer (8452 Å) with 2 nm resolution. Preliminary fluorescence measurements on a Perkin-Elmer fluorometer (LS50B) showed no observable fluorescence at room temperature for the PbI2 nanoparticles in solutions. TEM measurements were taken using a low resolution JEOL-100 CX (UCSC) and a high-resolution JEOL-200 CX transmission electron microscopes (Lawrence Berkeley National Laboratory). AFM and XPS measurements were performed in Prof. G.-y. Liu’s laboratory at Wayne State University. The transient absorption experiments were performed using a femtosecond Ti-sapphire laser system, involving a pumpprobe scheme, as described previously.3 Pulses of 40 fs duration (5 nJ/pulse energy) at a repetition rate of 100 MHz were generated and amplified in a Ti-sapphire regenerative amplifier using chirped-pulse amplification. The final output laser pulses of 200 fs duration with energy of 250 µJ/pulse, centered at 780 nm were generated at 1 kHz. This amplified output was doubled using a 1 mm KDP crystal to generate 390 nm pulses (20 µJ/ pulse), which were used to excite (pump) the sample. The sample was contained in a quartz cell with 1 cm optical path length. The remaining 780 nm light was used to generate a probe pulse in the wavelength region of 650-950 nm for monitoring the photogenerated electrons, using white light generation in a quartz window. The desired probe wavelength was selected by using an interference band-pass filter. The probe beam was split into a signal and a reference beam, which were detected by two silicon photodiodes and processed by a computer controlled, gated integrator in conjunction with an ADC. Pulse-to-pulse fluctuation of the laser beam was eliminated by normalizing the signal with respect to the reference for each laser pulse. Typically, 5000 laser pulses were averaged for each data point. The time delay between the pump and probe pulses was controlled by a translation stage. The pump and the probe beams were focused with a 10 cm focal length lens and crossoverlapped over a spot size of 0.5 mm2 in the sample before the focal point. The pump power was attenuated using neutraldensity filters to avoid generating any signal from the pure solvent, due to multiphoton ionization. The concentration of all the samples studied in the dynamics measurements was adjusted to an OD of about 1-1.5 at 390 nm. The dynamic features were independent of the concentration, and the signal size was linearly proportional to the sample concentration and laser excitation intensity. The photodecomposition of PbI2 mentioned earlier is relatively insignificant during each set of dynamics measurements and thus does not affect the dynamics data observed. The change in particle size allowed us to study the size dependence of the electron dynamics. Results Electronic Absorption Spectra and Surface Structure of Colloidal PbI2 Nanoparticles. One of the interesting features of layered iodide compound is the presence of well-resolved peaks in the absorption spectra of colloidal solutions. These peaks, which are considerably blue-shifted from the absorption peaks of the bulk, have been attributed either to preferred particle
3130 J. Phys. Chem. B, Vol. 103, No. 16, 1999
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Figure 2. Ground State electronic spectra of PbI2 nanoparticles in aqueous solution containing PVA aged under light for a week (solid) and in PVA film (dashed).
Figure 1. (A) Electronic absorption spectra of PbI2 nanoparticles in acetonitrile: aged in dark for one week (dotted), aged under light for one week (solid), and aged under light for three weeks (dashed). Inset: (a) fresh; (b) aged in dark for 1 day; (c) aged under light for 1 day; (d) aged under light for 2 days. (B) Electronic absorption spectra of PbI2 nanoparticles in propanol aged under light for a few days (solid line) and a few weeks (dashed line), and in butanol aged under light for few weeks (dotted line). The samples aged for three weeks under light were diluted by a factor of 3 for measuring the spectra in both (A) and (B).
sizes, termed “magic numbers” and/or to multiple excitation in a single particle in the quantum well according to the selection rules.15 Figure 1A shows the ground-state electronic absorption spectra of PbI2 nanoparticles in acetonitrile aged under light from one week to over a few weeks. The spectrum of PbI2 nanoparticles in acetonitrile completely aged (over a few weeks) consists of three well-resolved optical absorption bands with peaks at 242 nm (5.12 eV), 292 nm (4.25 eV), and 360 nm (3.44 eV). The inset of Figure 1A shows that the peak at 364 nm for the sample freshly prepared or aged under dark shifts to the blue upon aging under light. After 2 days under room light, the peak shifted to 362 nm. Further shifting to 360 nm occurred upon further aging for over a week. During the aging process, the particle size decreased from 30 and 60 nm (a bimodal distribution) to 3 nm. To determine the possible effect of solvent on the electronic absorption spectrum, PbI2 nanoparticles in various solvents were prepared, including water, aqueous solution containing PVA, 1-butanol, 2-propanol and ethanol. Figure 1B shows the groundstate electronic absorption spectra of PbI2 nanoparticles in 1-butanol and 2-propanol, aged under light from a few days to a few weeks. The spectra are very similar to those in acetonitrile, with the peak at 360 nm for the freshly prepared sample slightly blue shifted to 358 nm upon aging under light. During this aging process, the average particle size decreased from 6 to 3 nm. Figure 2 shows the ground-state electronic absorption spectra of PbI2 nanoparticles in aqueous solution containing PVA and in PVA film. In pure water without PVA, there is no absorption at wavelength longer than 300 nm and thus no PbI2 nanoparticles seem to be formed. With the presence of PVA in water, a new sharp excitonic peak at 414 nm appeares. To the blue of this band appear several higher energy bands that are broadened.
FTIR data on free-standing films of PVA and PVA film containing PbI2 nanoparticles are shown in Figure 3. The major difference between the two FTIR spectra is in the 1300-1500 cm-1 region, where two peaks present in the free-standing PVA film, 1425 and 1331 cm-1, are replaced by a strong peak at 1367 cm-1 in the PVA film with PbI2 nanoparticles. This is taken as an indication of strong interaction between PVA and the layers of PbI2. Figure 4A,B shows the low-resolution TEM measurements, revealing the particle sizes of PbI2 nanoparticles in acetonitrile aged for a few weeks (A) and for a few days (B). The average particle size is about 3 nm in (A). For (B) there is a bimodal distribution of 30 and 60 nm particles. Figure 4C shows the electron diffraction pattern from highly anisotropic layered structure of PbI2 in butanol sample aged under light for a few weeks. The picture clearly indicates that the particles grew with well-ordered hexagonal unit cell structure (possibly the most common 2H polytype PbI2 with D33d symmetry). From the electron diffraction pattern we calculated the lattice constants to be 5.4 and 6.5 Å, with about 10% uncertainty. These values are close to the values of bulk PbI2 crystals (4.6 and 6.98 Å).22,23 Figure 4D shows a high resolution (0.1 nm) TEM micrograph of PbI2 nanoparticles in butanol aged under light over a few weeks. The image is clearly from a number of particles present. One can see from the image that that the orientation of the PbI2 layers changes from one set of layers (which appear as parallel lines) to another; the distance between two adjacent layers is estimated to be 7.6 Å. Similar observations has been made previously in layered semiconductors like MoS2.15 Ultrafast Transient Absorption Measurements. Figure 5 shows the transient absorption decay profiles of photoexcited PbI2 nanoparticles in acetonitrile for samples aged for a few days (a) and for a few weeks (b) under light, probed at 720 nm following excitation at 390 nm. Figure 6 shows the time evolution on longer time scales for the same sample as used for Figure 5a. Two key features are worth noticing. First, following an instrument-response-limited rise (250 nm is most likely due to PbI2 nanoparticles rather than I3-. Assignment of Transient Absorption Signals. The transient absorption measurements seem to indicate the following: (i) the transient absorption decay profile is solvent or surface dependent but independent of particle size; (ii) the oscillation period observed at early times is also dependent on solvent but not size. On the basis of previous studies of a number of semiconductor nanoparticle systems,1-13 we expect that the transient absorption signal in the red to near-infrared region to originate from electrons in the conduction band, the excitonic state, or trap states. It is generally difficult to determine whether the electron is in the excitonic state or in the bottom of the conduction band since both are likely to be populated at room temperature and both should be relatively insensitive to surface or solvent variations. However, it is possible that the relaxation dynamics of these states are sensitive to surface changes due to coupling with trap states that are surface related. As discussed in detail in our early studies of AgI and Fe2O3 nanoparticles,11,12 based on the surface sensitivity of the decay dynamics and extremely low fluorescence, we suggest that the main contribution to the transient absorption signal observed in these systems is from trapped electrons. However, conduction band electrons (including electrons in the excitonic state) are likely to have some contribution to the signal as well at early times.2,10 The recent work by Lian et al. on the transient infrared studies of TiO2 nanoparticles seems to show strong evidence of important contribution of conduction band electrons to the transient infrared signal.10 As the relaxation evolves with time, the relative contribution of trapped electrons over conduction band electrons becomes larger. On the time scale of tens or hundreds of picoseconds, the signal is dominated by trapped electrons. In the present case of PbI2, the transient absorption signal is again primarily assigned to trapped electrons, with possible contribution from conduction band electrons at early times. This assignment is most consistent with observations of surface or solvent dependence and size independence of the relaxation dynamics and early time oscillations, extremely low fluorescence yield, and indications of a high density of trap states.5,19 Oscillations Observed at Early Time Dynamics. The oscillations observed at early times in the transient absorption profiles are not completely understood. Similar oscillations have been observed in semiconductor nanoparticle systems, such as PbS38 and CuCl.39 There is a possibility that the observed oscillations in PbI2 nanoparticles here are also due to quantum beats. However, since only one oscillation period is clearly observable due to the overall fast decay of the relaxation process, the assignment of the oscillations to quantum beats is possible but apparently not very convincing. Another possible and perhaps simpler explanation is that the early very fast decay followed by a quick rise is a reflection of dynamic processes that result in changes in the population or absorption cross section of the electron in going from one state to another. For example, if the initial signal has large contributions from electrons in the conduction band or shallow traps, as the electrons relax from the conduction band to trap states or from shallow trap to deep trap states, the signal can decrease to give rise to the very fast decay at early times. As electrons in these (deep) trap states further relax, the signal from electrons in the relaxed final state may increase, reflected as a rise time if there happens to be a larger absorption cross section for the final state at the probe wavelength. This is apparently a qualitative argument and a more quantitative model to fit the data is difficult
3136 J. Phys. Chem. B, Vol. 103, No. 16, 1999 at present time due to limited information about the states involved and relative absorption cross sections. It should be emphasized, however, that the oscillations observed are reproducible and well resolved above the S/N ratio. No such oscillations have been observed in our previous studies of a number of nanoparticle systems, including CdS, TiO2, AgI, Fe2O3, CuS, and PbS.2,11-13,25,26 A more definite explanation for the observed oscillations may be provided in the future when more knowledge is gained about the PbI2 nanoparticle system or better data can be acquired to clearly show oscillations with more than one period. Overall Electronic Relaxation Dynamics: Effects of Size and Surface. The effect of size on dynamic properties of charge carriers is still not well understood. Compared to bulk, two changes may be expected for semiconductor nanoparticles that could potentially affect the electron lifetime. First, as the size decreases, weaker electron-phonon interaction is expected due to the decrease of density of states for both the electron and phonons. Thus the electron lifetime is expected to increase with decreasing size. On the other hand, spatial confinement could lead to stronger electron-hole interaction, which increases the possibility of electron-hole recombination and decreases the electron lifetime. In addition, the lifetime may be affected by the surface properties due to a high density of trap states, which lead to significant nonradiative decays and shorten the electron lifetime. As the size becomes smaller, a larger percentage of surface atoms are present in the particle and a shorter electron lifetime may be expected. In most nanoparticles, as is probably the case for PbI2 here, the intrinsic size effect on electronic relaxation is difficult to determine because the surface effect dominates and the relaxation is typically very fast due to trapping and recombination facilitated by a high density of trap states. Previous studies have shown that a high density of surface trap states exists in PbI2 nanoparticles.5,19 Photoluminescence measurements of PbI2 nanocrystals embedded in porous silica showed the influence of surface states on the dynamic properties of various recombination processes.18 Surface-related defects are also very common in layered compounds like PbI2, because intralayer defects can effectively act as electron traps, which are equivalent to the surface defects in nanocrystals. The presence of such intralayer defects is again strongly dependent on the quality of the sample and the method of preparation. Thus, their contributions to the electronic relaxation process may vary. The size independence of decay dynamics in PbI2 nanoparticles and the absence of fluorescence at room temperature suggest that nonradiative decay through surface trap states is the dominant mechanism of electron relaxation. The time scale for electron trapping and chemical nature of trap states in most colloids are still disputable and are dependent on the quality of the samples or the decay mechanism. Previous studies of CdS and TiO2 colloids suggested that electron trapping occurred on the time scale of
