Nanotwinning and structural phase transition in CdS quantum dots
© Kumar et al.; licensee Springer. 2012
Received: 20 July 2012
Accepted: 16 October 2012
Published: 23 October 2012
Nanotwin structures are observed in high-resolution transmission electron microscopy studies of cubic phase CdS quantum dots in powder form by chemical co-precipitation method. The deposition of thin films of nanocrystalline CdS is carried out on silicon, glass, and TEM grids keeping the substrates at room temperature (RT) and 200°C by pulsed laser ablation. These films are then subjected to thermal annealing at different temperatures. Glancing angle X-ray diffraction results confirm structural phase transitions after thermal annealing of films deposited at RT and 200°C. The variation of average particle size and ratio of intensities in Raman peaks I2LO/I1LO with annealing temperature are studied. It is found that electron-phonon interaction is a function of temperature and particle size and is independent of the structure. Besides Raman modes LO, 2LO and 3LO of CdS at approximately 302, 603, and 903 cm−1 respectively, two extra Raman modes at approximately 390 and 690 cm−1 are studied for the first time. The green and orange emissions observed in photoluminescence are correlated with phase transition.
Colloidal semiconductors' nanocrystals (NCs) have been rigorously studied by various researchers because their unique physical properties are a function of their particle size. The bandgap observed in the absorption and emission spectra of semiconductor quantum dots (QDs) are blueshifted due to confinement of charge carriers. The optical properties of nanoparticles can also be determined by coupling of confined charges and confined phonons. The physical properties, especially the light emitting properties, change drastically as the size of the semiconductor materials become smaller. The optical and electrical properties of semiconductor NCs for applications in optoelectronic devices[1, 2] and biological fluorescence labeling[3, 4] are affected by quantum confinement when their typical dimensions are equal to or smaller than the Bohr radius of exciton. Cadmium sulfide, with a bulk bandgap of approximately 2.42 eV (approximately 512 nm) and exciton Bohr radius of approximately 2.85 nm, is a candidate for quantum-dot blue light-emitting diodes. It can also be used in photovoltaic devices. The microstructural characterization of NCs or QDs with size comparable to Bohr's exciton radius is useful to understand the light emission mechanism. Recently, our group reported the most common microstructural defects, i.e., twin structure, stacking faults, and grain boundaries in CdS QDs for the first time. The surface and structural defects are expected to have important effects on the physical properties, particularly the optical properties of the QDs.
Thin films of wide bandgap II-VI semiconductors are of considerable interest as their emissions cover the technologically attractive blue and green spectral regions. In particular, CdS thin films attracted more attention because their bandgap emission is expected to lie very close to the highest sensitivity of the human eye, i.e., green light. The thin films of CdS quantum dots seek wide applications in photonic devices like laser, LEDs, etc. Ullrich and his group demonstrated optically pumped laser action in pulsed laser deposition (PLD)-grown CdS thin films[7, 8]. Artemyev and Nanda et al. reported electroluminescence and photocurrent studies in devices fabricated using CdS nanocrystals. Nizamoglu et al. fabricated white LEDs using CdS quantum dots hybridized on near-UV LEDs. Various methods have been used by different researchers to synthesize CdS nanocrystals thin films[12–17]. It has been shown that PLD is a versatile technique to maintain stoichiometry of film because of rapid temperature rise (>1011 K/s) produced by focused pulsed laser beam on the target. Growth of high quality films at a relatively low substrate temperature by PLD is possible because high-energy atoms and ions in the laser-induced plasma plume create a higher surface mobility. A lot of work has been reported on PLD grown CdS films investigating the effect of various parameters such as substrates, substrate temperature, laser fluence, laser wavelength, etc.[14, 19–26]. Still, there is a need of further studies on PLD-grown films for the development of deeper understanding of their structures for future applications. To the best of our knowledge, the room temperature (RT) deposition of CdS thin film was never reported before. The thermal annealing-induced phase transition has been studied in thin films deposited by different routes[27–29], but not studied in PLD-grown films.
In this letter, we report on the studies of properties of CdS thin films grown by PLD, keeping the substrates at RT and 200°C, and the annealing effects on the structural and optical properties of the films. The variation of average particle size and ratio of intensities of Raman peaks I2LO/I1LO are studied with respect to the annealing temperature. It seems that electron-phonon interaction is a function of temperature and particle size, irrespective of the structure. Two extra modes in Raman spectra have been identified for the first time. These are verified by low frequency Raman studies.
Thin films of CdS quantum dots are deposited by laser ablation of a target prepared by pressing and sintering the chemically synthesized CdS QDs powder. The synthesis of CdS quantum dots in powder form is reported elsewhere. For PLD, using ultraviolet laser source, a pulsed excimer KrF laser (Lambda Physik, Compex Pro 201, Coherent Inc., CA, USA) operating at 248-nm wavelength has been used. The pulse width of 10 ns and energy of 300 mJ per pulse have been used. The laser beam with a repetition rate 10 Hz is focused onto a rotating target mounted at an oblique angle of 30°. The distance between target and substrate is kept as 5.5 cm. The films are deposited on single-crystal (111) n-type silicon wafers, glass and carbon-coated Cu grids (for TEM) at two different temperatures: (1) RT and (2) 200°C inside a clean stainless steel vacuum chamber with a background pressure of 5 × 10−6 mbar. The silicon and glass substrates are cleaned using a standard process that involves boiling in trichloroethylene followed by rinsing with deionized water. The CdS deposition rate in this configuration is about 0.025 nm per pulse. The thickness of deposited film is about 250 nm. To study the post-annealing effect on structural and optical properties of the films, thermal annealing is carried out at different temperatures 300°C to 450°C for 3 h in Ar environment.
Glancing angle X-ray diffraction (GAXRD) studies are carried out at an angle of 1° using Bruker D8 diffractometer (Bruker AXS GmbH, Germany; Cu Kα radiation, λ = 1.5406 Å) and micro-Raman spectroscopy using Renishaw Invia Raman microscope (Renishaw plc, Gloucestershire, UK) with 514-nm excitation wavelength of an Ar ion laser. The low frequency micro-Raman scattering measurements were performed in the backscattering geometry using a Jobin Yvon T64000 triple monochromator (NJ, USA) with a Coherent INNOVA 99 Ar+ laser (514.5 nm) equipped with a charge-coupled device detector. The samples are examined by transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) using Tecnai G20-Stwin operating (FEI Company, Shanghai, China) at 200 kV with point resolution of 1.44 Å, line resolution of 2.32 Å, and line-type super twin lenses. The films deposited on glass substrates are analyzed using UV-vis absorption spectroscopy (Hitachi 3300 UV/visible spectrophotometer; Hitachi High-Technologies Corporation, Tokyo, Japan). Photoluminescence (PL) spectroscopy studies are carried out at room temperature using HORIBA Jobin Yvon LabRAM 800 HR (NJ, USA) with excitation wavelength at 325nm from He-Cd laser.
Results and discussion
Figure3 shows that as-grown film at substrate temperature of 200°C is also in mixed phase of cubic and hexagonal structures but with different orientation of planes. The diffraction peak at 26.64° is assigned to (111) plane of the cubic structure, whereas the peaks at 36.8° and 48.1° are assigned to (102) and (103) planes of the hexagonal structure, respectively. It is clearly seen from Figure3 that comparatively stable hexagonal phase starts to dominate over the mixed phase at annealing temperature of 350°C. Complete hexagonal phase occurs at 400°C. The diffraction peaks at 24.8°, 26.48°, 28.12°, 43.64°, 47.8°, and 51.9° correspond to the (100), (002), (101), (110), (103), and (112) planes (PCPDF WIN 751545) of the hexagonal structure with lattice parameters a = 4.12 Å and c = 6.724 Å for the film annealed at 350°C. There is one additional diffraction peak at 31.6° corresponding to (002) plane of the hexagonal phase of Cd. It is also clear from Figure3 that there is a phase transition from hexagonal to cubic phase for the film annealed at 450°C.
Cadmium sulfide has C6V symmetry with four atoms per unit cell. Group theory predicts that there are nine optical branches at the zone center. These optical branches can be classified as one symmetric A1 and one doubly degenerate E1 which are both Raman and infrared active, two doubly degenerate E2 branches which are Raman active only, and two antisymmetric with respect to the twofold and sixfold axes (B1) ‘silent modes’ inactive in both infrared absorption and Raman scattering[30, 31].
The influence of annealing in Ar environment on PLD-grown CdS thin films at two different temperatures, (1) RT and (2) 200°C has been studied. The annealing-induced phase transition is observed in both types of films with different features. The structural phase transition is correlated with variation in particle size, bandgap, intensity ratio of 2LO to 1LO (I2LO/I1LO) Raman peaks, asymmetry in 1LO mode, and PL results. It may be concluded that in either of the phases viz. mixed, cubic or hexagonal, the ratio I2LO/I1LO increases as the temperature and particle size increase, and electron-phonon interaction is a function of temperature and particle size, irrespective of the structure. The existence of the two extra Raman active modes at approximately 390 and 690 cm−1 is observed using low frequency Raman measurement.
PK is a research scholar under the supervision of AA, an assistant professor at the Department of Physics, Bareilly College, Bareilly, India. NS is research associate working with DK at Inter University Accelerator Centre, New Delhi, India. RC and VG are professors at Indian Institute of Technology, Roorkee and University of Delhi, respectively.
The authors are thankful to Dr. RP Singh, the principal of Bareilly College, Bareilly for providing the necessary facilities and moral support. The assistance provided by Mr. Ravish Jain and Mr. Paritosh Dubey, Indian Institute of Technology, Roorkee during thin film deposition is highly appreciated. The help received from Mr. Pawan Kulariya and Dr. Vinod Kumar, IUAC, New Delhi in GAXRD measurements and Raman measurements is gratefully acknowledged. The authors are also thankful to Ms. Kajal Jindal from the University of Delhi for the PL and low frequency Raman measurements.
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