Time-resolved photoluminescence spectroscopy of

Time-resolved photoluminescence spectroscopy of excitons in layered semiconductor PbI2 nanoclusters Yu. P. Gnatenko, P. M. Bukivskij, Yu. P. Piryatinski, A. P. Bukivskii, P. A. Skubenko et al. Citation: J. Appl. Phys. 112, 093708 (2012); doi: 10.1063/1.4764315 View online: http://dx.doi.org/10.1063/1.4764315 View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v112/i9 Published by the American Institute of Physics.

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JOURNAL OF APPLIED PHYSICS 112, 093708 (2012)

Time-resolved photoluminescence spectroscopy of excitons in layered semiconductor PbI2 nanoclusters Yu. P. Gnatenko,1 P. M. Bukivskij,1 Yu. P. Piryatinski,1 A. P. Bukivskii,1 P. A. Skubenko,1 and R. V. Gamernyk2 1

Institute of Physics of NASU, Prospect Nauky 46, Kyiv 03028, Ukraine Lviv National University, 8 Kyrylo and Mefodiy Street, 29005 Lviv, Ukraine

2

(Received 2 August 2012; accepted 9 October 2012; published online 5 November 2012) We studied the dynamics of excitons excited in layered semiconductor PbI2 nanoclusters (NCLs) using time-resolved photoluminescence (TRPL) spectroscopy. TRPL spectra reveal formation of self-trapped excitons (STEs). It was shown that the formation of the STEs for larger [more than the Bohr radius of exciton in bulk PbI2 (RB ¼ 1.9 nm)] NCLs occurs in sub-nanosecond time scale, while in the case of small NCLs with sizes about RB, it takes place in nanosecond scale. The effective energy transfer from the small to the larger semiconductor NCLs, which arises from dipole-dipole intercluster interactions, takes place in sub-nanosecond scale. We demonstrate that the STEs are stable states and they define effective radioluminescence of the investigated C 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4764315] Pb1XCdXI2 alloys. V I. INRODUCTION

II. EXPERIMENTAL DETAILS

At present, there are a number of works available devoted to the synthesis of the lead iodide nanocrystals (NCs) and study of their optical properties. This semiconductor crystal is a very promising high-sensitive noncooled radiation detector material for x- and c-rays suitable for biomedical and industrial imaging applications.1,2 PbI2 is a layered direct semiconductor with a repeating unit of a hexagonally closed-packed layer of lead ions sandwiched between two layers of iodide ions.1–4 Recently,1,5 we observed room-temperature stationary photoluminescence (PL) of the PbI2 nanoclusters (NCLs), which are naturally formed in Pb1XCdXI2 alloys as a result of non-isoelectronic substitution at cation sites. The layered structure of these alloys facilitates formation of quantum-confined nanostructure. Thus, these alloys present a three-dimensional semiconductor material with randomly distributed NCLs. The presence of such NCLs stimulates appearance of a broad PL band with a large Stokes shift, which, as it has previously been shown,1,6,7 is apparently caused by formation of self-trapped excitons (STEs) in PbI2 NCLs. However, the dynamics of excitonic states with the formation of STE states in various sizes anisotropic NCs have not yet been experimentally investigated. The aim of this work is thus to study the dynamics of excitons in the layered PbI2 NCLs that may help determine the nature of the intense photo- and radioluminescence spectra of these crystals at room temperature. For this purpose, we have performed time-resolved PL spectra measurements as well as measurement of the radioluminescence spectra for these crystals. It should be noted that earlier the timeresolved PL measurements for layered semiconductor NCLs have not been carried out. Understanding of this dynamics is important, since the rate of the emission intensity of PbI2 NCL assemblies determine the output of these scintillator detector materials.

The Pb1XCdXI2 alloys were grown by the Bridgman technique. The synthesis of the crystals was performed by means of direct alloying of the constituents in sealed quartz ampoules under vacuum. The sample composition was determined by the electron probe microanalysis. We have characterized the surface morphology of these crystals using JEOL JSM-T220A scanning electron microscopy. Fig. 1 shows SEM images of the surface of the investigated crystals, where the insert shows the SEM image for PbI2 crystal. In these measurements, a back-scattered electron (BSE) regime was used, i.e., the high-energy electrons are reflected or back-scattered out of the specimen interaction volume by elastic scattering interactions with specimen atoms. Heavy elements backscatter electrons more strongly than light elements. Thus, in the former case, the image is brighter than in the latter. Consequently, BSE allows us to detect the contrast between the areas with different chemical compositions. For the investigated Pb1XCdXI2 alloys, the atomic number of Pb atoms (82) is considerably larger than that for Cd atoms (48). Therefore, the bright spots in the SEM images of this alloy correspond to the formations with the participation of Pb atoms. X-ray diffraction measurements performed using STOE STADI P diffraction system showed no metallic inclusions of Pb or Cd atoms and the presence of PbI2 and CdI2 crystal phases. This indicates that the bright spots in SEM image of Pb1XCdXI2 alloys correspond to NCs of PbI2 of various sizes from several nm up to about 200 nm. The resistivity of the crystal samples was estimated to be about 1014 Xcm. The crystals were excited by a nitrogen pulse laser (k ¼ 337.1 nm) with a pulse duration of 9 ns and the peak power density of 5 kW/cm2.8–10 It should be noted that the technique of time-resolved photoluminescence (TRPL) spectroscopy is similar to the technique of time-resolved photoelectric spectroscopy described by us in details8 and is based on the stroboscopic recording system with a time window of 0.1 ns.

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FIG. 1. SEM image of Pb1XCdXI2 (X ¼ 0.70) showing the crystal surface morphology. In the insert, SEM image of PbI2 crystal is presented.

III. RESULTS AND DISCUSSION

The room-temperature time-resolved PL spectra of the investigated Pb1 XCdXI2 (X ¼ 0.70) alloys are presented in Fig. 2. The spectrum with sd ¼ 0.7 ns (curve 1), where sd is the delay time of the registration of signal, includes an intensive long-wavelength (LW) and a weak short-wavelength (SW) bands in the spectral ranges 500–750 and 400–450 nm, respectively. In this case, the excitation takes place on the ascending part of the laser pulse.8 The latter band (enlarged) is also presented in the inset of Fig. 2. Previously,1 it was shown that the band corresponds to the emission of excitons excited in the PbI2 NCLs (for the bulk crystal, the maximum of PL exciton band is located around 500 nm (Ref. 1)), which are formed in the Pb1XCdXI2 crystalline matrix within the crystal layers. The crystal structure of PbI2 is the same as that of CdI2 and corresponds to 4 H polytype.1,3,4 However, in the case of Pb1XCdXI2 alloys, a nonisoelectronic system is formed because the valence electrons of Pb and Cd atoms belong to the different electronic configurations, namely (6s26p2) and (5s2), respectively.1,3,4 The valence band and the conduction band of PbI2 are composed mainly of electronic states of Pb2þ ions. Thus, in this crystal, the appearance of excitons is due to the excitation of Pb2þ ions and the transfer of energy between them. There are two exciton states of C3 and C1 symmetry, which are related to the 3P1- and 3 P0-states of isolated Pb2þ ions, respectively. The lowest C1-state is essentially dipole-forbidden, and for excitonic C3 state, the optical transition is partially dipole-allowed.3 The structure of the SW band reflects the emission of free excitons for the PbI2 NCLs of different sizes. In the effective-mass approximation, assuming that an infinite, three-dimensional potential well confines both electrons and holes within the boundaries of the layered NCL, it was found

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FIG. 2. (a) Time-resolved PL spectra of Pb1XCdXI2 alloys at T ¼ 300 K. Curves 1-3 correspond to the delay time of 0.7, 20, and 800 ns, respectively. Curve 4 corresponds to the stationary PL spectrum. (b) In the insert, the PL spectrum of the delocalized excitons in the PbI2 NCLs is presented.

that the blueshift of the exciton energy as a result of the quantum-confined effect has the form11 E ¼ h2 =2lxy ½n2x p2 =L2x þ n2y p2 =L2y þ h2 =2lz ½n2z p2 =L2z ; (1) where x and y are the coordinates in the layer plane, z is the coordinate perpendicular to the layer plane, ni ¼ 1 for the lowest excitonic transition, Li are the sizes of NCLs in the respective directions, and lxy and lz are the reduced effective masses of excitons within the layer plane and perpendicular to it, respectively. Taking into account the measured blueshift of exciton energy for the NCLs, the values of lxy ¼ 0.32 m0 and lz ¼ 1.4 m0 (Ref. 12), and using Eq. (1), we have determined the average lateral sizes Lxy of NCLs for Lz ¼ 1.4 nm (two layers) and Lz ¼ 2.1 nm (three layeres). These data are presented in Fig. 3(a). We assume that the NCLs with Lz ¼ 1.4 nm are the most probable because for 4 H polytype the unit cell includes two crystal layers. The intensity of different components in the PL spectrum of the SW band lets us obtain the histogram of NCLs size distribution in Pb1 XCdXI2 (X ¼ 0.70), which is shown in Fig. 3(b). These data indicate that the average lateral size of the NCLs is about 2.5 nm, which is very close to one of the “magic” numbers for PbI2 NCL (2.9 nm (Ref. 12)). Thus, the NCLs observed in the investigated crystalline matrix present the quantum discs with the average lateral size of 2.5 nm and the height of 1.4 nm. The intensive LW band in Fig. 2 is caused by the PL of the STEs. According to Ref. 13 for PbI2, these states can exist at the stretching Pb–I bonds on the NCL surface. It is seen that for sd ¼ 0.7 ns, the band maximum is about 630 nm. The spectra also show that the PL of free excitons disappears


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FIG. 3. (a) Dependence of the blueshift of the exciton energy on the average lateral dimension of NCLs, obtained using Eq. (1). (b) Histogram of NCLs size distribution in Pb1XCdXI2 (X ¼ 0.70).

when the delay time is larger than 0.7 ns. At the same time, the increase in PL intensity occurs in the spectral range around 600 nm. This reduces the half-width of the band and shifts its maximum to the SW region up to 585 nm during the next 800 ns. The formation of the STE states in the PbI2 NCLs may be considered using the schematic configuration coordinate diagram presented in Fig. 4(a), where the ground (G), normal (delocalized) excitonic (E), and STE states are shown. The configuration coordinate Q corresponds to the stretching of the Pb–I bond. For Q0, the ground and the first excited states are delocalized (inner wells) while for Q Q1, the electron-hole pair is localized on a single bond (outer well). The obtained results indicate that the STE states in the PbI2 NCLs are stable states while the excitonic E-state is a metastable state, i.e., the outer well is lower than the inner well. At room temperature, the STE states are mainly formed due to the thermal activation over the barrier of the delocalized excitons.14 Analysis of the obtained results indicates that the PL near 630 nm is caused by the formation of the STEs with the participation of larger NCLs (Lxy 2RB). The excitons excited in the NCLs with the average Lxy ¼ 2.5 nm (< 2RB) are responsible for the formation of the STEs in the region about 600 nm. The structure of the PL band observed in this spectral region reflects the participation of excitons excited in the NCLs of different sizes. The thermal activation rate of the delocalized excitons for the larger NCLs into the STE states is shorter than the one for the smaller NCLs.

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FIG. 4. (a) Schematic configuration diagram of a Pb–I bond at the NCL surface, which shows excitation from the ground state (G) into the first excited (exciton) state (E) and PL from the STE to the ground state. (b) Dependence of the population of STE state (N2) as a function of DU/kT for different values of x1/x2, where x1 and x2 are vibration frequencies in the G and STE states, respectively.

Besides, the F€orster ET between the NCLs of various sizes as a result of the dipole-dipole interaction can take place.15,16 The F€orster rate for the semiconductors with a direct gap is mainly determined by the spectral overlap between the emission and absorption spectra of the NCLs.15 As a result of the ET from the smaller (donor) to the larger (acceptor) NCLs, the population of excitons in the inner well for the latter NCls will increase. This will enhance the efficiency of their thermal excitation to the STE states. The observation of the SW PL band for sd ¼ 0.7 ns indicates that there are NCLs which do not participate in ET, which is likely due to the absence of efficient resonant acceptor NCLs near these donor NCLs.16 In the thermal equilibrium at temperature T, there are fractional populations between the inner (N1) and the outer (N2) wells, which are determined by the following transfer function:14 N2 ¼ ðx1 =x2 Þ eDU=kT =½1 þ ðx1 =x2 Þ eDU=kT ;

(2)

where DU is the energy difference of the vibrational ground states of the upper wells as shown in Fig. 4(a), x1 and x2 are the vibrational frequencies in the wells, and N1 ¼ 1 – N2. Fig. 4(b) shows the dependence of N2 as a function of DU/kT for different values of x1/x2. It can be seen that N2 1 for DU/kT 5. Since the stationary PL spectrum manifests only the STE emission, it means that the


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population of the inner well has been completely transferred to the outer well. Therefore, the gap between the two wells DU 5 kT (135 meV), i.e., for the observed PbI2 NCLs, the STE state is really a stable state. Another mechanism of population transfer from the inner well into the outer well is quantum-mechanical tunneling through the barrier DE,14 which may be effective at low temperatures when the thermal activation mechanism is very unlikely. The tunneling occurs with participation of the double-well vibrations, i.e., the vibrations in the inner and outer wells for which the energy levels are the same. For the inner well, the lowest double-well vibration is the ground vibronic state, while for the outer well, it corresponds to the one of the excited vibronic states. It is expected that for the PbI2 NCLs, the tunneling process is slower than the thermal activation.14 In Fig. 5(a), the low-temperature (4.5 K) timeresolved PL spectra are shown, where the emission of the STEs is observed only for the stationary PL spectrum. The maximum of the STE PL band is shifted to the LW region in comparison with the spectrum at the room temperature that is due to the emission of the STEs from the lowest vibronic state of the outer well.13 The observation of free excitons for NCLs at T ¼ 4.5 K in the stationary PL spectrum together with the emission of STEs is due to the fact that the population exhibits oscillations between the two wells caused by tunneling.14 The PL about 500 nm is caused by the emission of the free excitons for NCLs of large sizes (considerably more than RB). As it can be seen from Fig. 1, such NCLs have been observed in the investigated crystals by scanning

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electron microscopy. It should be noted that for these NCLs, the blueshift of the exciton energy as a result of the quantum-confined effect is not observed. Therefore, the energy caused by the emission of free excitons for these NCLs and for bulk PbI2 crystals is the same. Since the properties of lead iodide are suitable for the development of scintillator detector materials, we also performed measurement of their room-temperature radioluminescence using x-ray excitation, which is shown in Fig. 5(b). The shape and energy maximum of this emission band are similar to the ones of the broad PL band presented above. It means that the nature of both bands is also the same. Thus, the Pb1 XCdXI2 alloys can be considered as new scintillator materials, where the radioluminescence is determined by the emission of STEs in the layered semiconductor NCLs, and can be named bulk-nanostructured scintillators (or nanoscintillators). It should be noted that these nanoscintilators are strongly radiation-resistant since as it was established in this work, the decrease of their emission during x-ray irradiation (about 30 min) was smaller than 3%. IV. CONCLUSIONS

In conclusion, the time-resolved and stationary PL spectra of PbI2 NCLs in a Pb1 XCdXI2 alloys have been studied. The obtained results have been explained using the model of self-trapped excitons on Pb–I bonds at the nanocrystal surface, which are formed as a result of the thermal activation or the quantum tunneling of the delocalized excitons through the barrier at room and liquid helium temperatures, respectively. The processes are rapid and occur on subnanosecond and nanosecond time scale and their rate depends on the NCLs sizes. It was shown that the investigated materials have very intense room-temperature PL and radioluminescence due to the existence of disclike PbI2 NCLs with average lateral size about 2.5 nm and the height of two layers. Therefore, for these NCLs, there exist many surface Pb–I bonds where the STEs are formed. The further increase of the emission intensity of such nanoscintillator materials is possible by optimizing the size distribution of NCLs in the Pb1 XCdXI2 alloys and by means of thermoelectric cooling. The cooling of the NCLs allows us to decrease the nonradiative recombination from STE states into the ground state by tunneling through the barrier DE* (see Fig. 4(a)). Thus, our results may pave the way towards a new class of effective scintillator materials based on the emission of the STEs in the semiconductor NCLs. ACKNOWLEDGMENTS

The research has been supported by the National Academy of Sciences of Ukraine (Grants Nos. BC-139-15 and B-146-15). We sincerely thank Dr. V. Ya. Degoda and Dr. A. Sofiienko for measuring of the x-ray spectrum. 1

FIG. 5. (a) Time-resolved PL spectra of Pb1XCdXI2 alloys at T ¼ 4.2 K. Curves 1-2 correspond to the delay time of 0.7 and 15 ns, respectively. Curve 3 corresponds to the stationary PL spectrum. (b) The radioluminescence spectrum of Pb1 XCdXI2 alloys at T ¼ 300 K.

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