- Email: [email protected]
nanocrystalline structures prepared via a thermal evaporation method
Materials Science in Semiconductor Processing 26 (2014) 87–92
Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Optical properties of CdS micro/nanocrystalline structures prepared via a thermal evaporation method M.A. Mahdi a,n, J.J. Hassan a, S.J. Kasim a, S.S. Ng b, Z. Hassan b a
Basrah Nanomaterials Research Group (BNRG), Physics Department, College of Science, Basrah University, Basrah, Iraq Nano-Optoelectronics Research and Technology Laboratory (N.O.R.), School of Physics, University Sains Malaysia, 11800 Penang, Malaysia b
a r t i c l e i n f o
Keywords: CdS nanostructure Thermal evaporation Photoluminescence spectra
abstract Cadmium sulfide (CdS) micro/nanocrystalline structures were fabricated by thermal evaporation in the absence of a catalyst on ITO-coated glass and Si(100) substrates. The samples prepared at 520 1C showed two structures of CdS: nanowires (NWs) and necklace-like microstructures grown on Si(111) and ITO substrates, respectively. The crystalline structures of the grown CdS necklace-like microstructures and NWs were studied using XRD analysis. The CdS necklaces and NWs structures were formed with hexagonal (wurtzite) structure. Two emission bands (green and red) were observed in the photoluminescence spectra of the prepared samples. The green PL emission in both structures showed a strong blue-shift compared with the optical band gap of CdS. Raman spectra of the grown CdS micro/nanocrystalline structures show redshift in the fundamental (1LO) and overtone (2LO) peaks location compared with Raman bands of bulk CdS. & 2014 Elsevier Ltd. All rights reserved.
1. Introduction Nanomaterials have recently received much attention because of their unique physical, chemical, and mechanical properties. The control on the fundamental properties of materials, such as magnetic, optical, and electrical properties, can be achieved without altering their chemical composition by controlling the size, structure type, and surface states of the nanocrystalline materials [1]. Wide band gap II–VI semiconductors have been studied intensively for many years because of their great potential for a variety of applications, especially in optoelectronic devices [2,3]. Cadmium sulfide (CdS) is one of the most important II–VI semiconductors, with a direct band gap of 2.42 eV at room temperature [4,5]. CdS is extensively used in optoelectronic devices because it tunes emission in the visible-
n
Corresponding author. Tel.: þ964 778686096. E-mail address: [email protected] (M.A. Mahdi).
http://dx.doi.org/10.1016/j.mssp.2014.04.015 1369-8001/& 2014 Elsevier Ltd. All rights reserved.
light range; it is also widely used to fabricate hetrojunction solar cells and optical sensors. Various growth methods are used to fabricate CdS nanowires (NWs), including direct current electrodeposition [6], solvothermal synthesis [7], and thermal evaporation [8]. Among these methods, thermal evaporation is used to grow single crystalline CdS micro-rod and nanosheet for high speed photodetction devices [2,3]. Single crystalline semiconductors have received much attention in the fabrication of high-quality optoelectronic devices because of their unique optical and electrical properties [9]. In this work, CdS micro/nanocrystalline structures were fabricated by thermal evaporation. Surface morphology and crystalline structure as well as the optical properties of the prepared samples were investigated. 2. Experimental CdS micro/nanocrystalline structures were fabricated by thermal evaporation in the absence of a catalyst. In this work, indium tin oxide (ITO) glass (1.5 cm 2.5 cm) and
88
M.A. Mahdi et al. / Materials Science in Semiconductor Processing 26 (2014) 87–92
Si(1 0 0) were used as substrates. CdS powder (1 g; purity, 99.999%; Fluka Chemical Co.) was placed in an alumina boat and fixed at the center of a quartz tube in a furnace. The ITO glass substrate was fixed at the distance of 25 cm from the center. One end of the quartz tube was connected to a gas source that supplied 50 sccm of argon (Ar) gas as the transport gas. The furnace temperature was then raised gradually to 900 1C within 30 min at 30 1C/min. This temperature was maintained for 2 h. During the deposition, the ITO glass and Si(1 0 0) substrates temperature was 520 1C. Once the deposition was completed, the furnace was cooled to room temperature under Ar gas flow. The morphology of the prepared CdS micro/nanostructures was characterized using Scanning Electron Microscopy (SEM; model JSM-6460LV). The structural properties of the grown CdS micro/nanostructures were examined using X-ray diffraction (XRD) (X'Pert Pro MPD X-ray diffractometer, PANalytical Company) with CuKα radiation. The optical absorption of the prepared CdS nanocrystalline structures was conducted using a Shimadzu UV–vis spectrophotometer model UV-18s00. The samples were removed from the substrates and immersed in water to measure the optical absorption. Room temperature photoluminescence (PL) and Raman spectra were obtained by a Horiba Jobin Yvon HR 800 UV equipment which uses 325 nm, 20 mW HeCd and 514.5 nm, 20 mW Ar ion laser light for excitation, respectively. 3. Results and discussion 3.1. Surface morphology Fig. 1 shows the SEM images and EDS spectra of the CdS nanowires (NWs) prepared via thermal evaporation on ITO-coated glass and Si(1 0 0) substrates. The sample prepared at 520 1C showed two structures of CdS: NWs and necklace-like microstructures. The CdS NWs were grown via solid–vapor–solid (SVS) mechanism on the substrate surface. The grown NWs contain nodes along the wire length (shown in Fig. 1B), which served as growth centers for the necklace-like microstructures. Fig. 1C shows the CdS NWs grown on Si substrate at 520 1C. Unlike the CdS structures on the ITO–glass substrates, the NWs appeared homogenous and node-free. The effect of the substrate on NW morphology and shape starts from the initial stage of growth. The substrate type has an important function in CdS micro/nanostructures growth. The EDS results showed that the Cd/S ratios were 1.19 and 1.11 for the CdS necklaces grown on ITO–glass and CdS NWs prepared on Si wafer, respectively. Fig. 2 shows a schematic for the suggested growth mechanism of the CdS necklace microstructures. 3.2. Crystalline structure The crystalline structures of the grown CdS necklacelike microstructures and NWs were studied using XRD analysis. The CdS necklaces and NWs structures were formed with hexagonal (wurtzite) structure, as shown in the XRD patterns (Fig. 3A and B) compared with a standard XRD database (PDF-4, 00-0010780). The lattice constants a
and c for the hexagonal structures were calculated using the following equation [10]: ! 2 2 2 1 4 h þ hk þ k l ¼ ð1Þ þ 2; 2 2 3 a c ðdhkl Þ where dhkl is the interplanar spacing of the atomic planes, and h, k, and l are Miller indices. The prepared structures were 4.14 and 6.72 Å, corresponding to the standard data of the bulk CdS material. The preferred orientation (Phkl) of the prepared CdS nanostructures was determined using the flowing formula [11]: P ðhklÞ ¼
I ðhklÞ ∑I 0ðhklÞ I 0ðhklÞ ∑I ðhklÞ
;
ð2Þ
where I(hkl) and I0 (hkl) are the integrated intensities of the experimental and random (simulated) XRD diffractions, respectively. For a random orientation, Phkl equals one. For a particular (hkl) plane with a preferred orientation, Phkl is greater than one. The necklace structure had a P002 of 1.53 and P101 of 1.07. These results indicate that plane (002) is the preferred orientation of the CdS nanostructures. In addition, the preferred orientation of the CdS NWs was along the (002) plane, where the P002 value was 3.64 compared with the 0.77 for the (101) plane. 3.3. Optical properties 3.3.1. Optical absorption The optical absorption of CdS necklaces and CdS NWs was investigated at room temperature over the wavelength of 400–800 nm to evaluate the forbidden energy gap (Eg). Fig. 4A shows the ultraviolet–visible (UV–vis) absorption spectra of the CdS necklace-like microstructures and NWs, respectively. The CdS NWs showed blueshift of the absorption edge compared with the absorption edge of CdS necklaces which can be related to the quantum size effect of the CdS NWs. The optical band gap Eg of direct semiconductor can be estimated from the following relationship [1]: α ¼ Aðhν Eg Þ1=2 ;
ð3Þ
where A is a constant and hν is the energy of the illuminating photon. For the direct band gap, Eg, which can be calculated from the linear portion of the curve, is extrapolated to (αhυ)2 ¼ ¼0 when (αhυ)2 is plotted as a function of hυ. The optical band gap values of the grown CdS necklacelike microstructures and NWs estimated from Fig. 4B are 2.44 eV, and 2.59 eV, respectively. 3.3.2. Photoluminescence spectra Fig. 5A and B shows, respectively, the PL spectra of the CdS necklaces and NWs structures prepared via thermal evaporation. Two emission bands were observed in both spectra. The necklace structure emitted at 484 and 653 nm, whereas the NW emission was located at 478 and 557 nm. The green PL emission in both structures showed a strong blue-shift compared with the optical band gap of CdS (512 nm). This shift in the emission band may be attributed to the quantum size effect of the CdS
89
Intensity (a.u.)
M.A. Mahdi et al. / Materials Science in Semiconductor Processing 26 (2014) 87–92
Intensity (a.u.)
Energy (KeV)
Cd: 52.65% S: 47.35%
Energy (KeV) Fig. 1. SEM images and EDS spectra of CdS nanostructures prepared via thermal evaporation: (A, B) CdS necklace micro-structures on ITO-coated glass and (C) CdS nanowires on Si wafer substrate.
NWs and the surface states. Moreover, the main emission peak intensity of the necklaces and NW structures at 484 and 478 nm, respectively, was strong, indicating that electro-hole recombination occurred near the band edge. Furthermore, the emission band of the NWs was sharper than that of the necklace structure. These results may be attributed to the following reasons. First, the NW
dimension (diameter) was less than that of the necklace structure that contained CdS grains in micrometer scale. Second, the Cd/S ratio affected the emission band intensity and location. The Cd/S ratio for the necklace and NW was 1.11 and 1.19, respectively. Thus, the emission intensity of the NW structure was higher because of its shape and dimensions. The intensity ratio of the main emission to
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M.A. Mahdi et al. / Materials Science in Semiconductor Processing 26 (2014) 87–92
Cd+S vapor
Cd+S atoms aggregator on the nodes
Formation of necklace micro-structure Fig. 2. Schematic of the suggested growth mechanism of the CdS necklace micro-structures.
Fig. 3. XRD patterns of CdS nanostructures prepared via thermal evaporation: (A) CdS necklace micro-structures on ITO-coated glass and (B) CdS NWs on Si wafer substrate.
that of defects peak (Imain/Idefect) of the necklace and NW structure was 2.5 and 5.9, respectively. The broad emission band of the necklace and NW around 653 and 557 nm (yellow band), respectively, can be associated with the structure defects or surface-trap emission. Excess Cd in CdS leads to the formation of an acceptor level within the CdS band gap. Thus, the yellow band may be attributed to the transition of the Cd-interstitial donors to the valence band [12]. However, Ahmad-Bitar [13] suggested more reasons for the appearance of the yellow emission band. He attributed this appearance to recombination via surface localized states, transition from ICd to the valence band, and transition from the interstitial Cd–Cd vacancy complex that acts as a donor to an accepter level. The intensity ratio of the band-edge emission to the defect emission (IBE/ID) was 2.46. In addition, the emission band in the red region
Fig. 4. (A) Optical absorption of CdS necklace micro-structures and CdS NWs and (B) plot of (αhν)2 vs. (hν) of CdS necklace micro-structures and CdS NWs.
(700–900 nm) disappeared, indicating the absence of sulfur vacancies that act as deep traps for electrons in the conduction band. This absence resulted in the red luminescence of CdS through the transfer to surface molecules or pre-existing trapped holes [14].
M.A. Mahdi et al. / Materials Science in Semiconductor Processing 26 (2014) 87–92
91
Fig. 6. Raman spectra of (a) CdS necklace micro-structures on ITO-coated glass and (b) CdS NWs on Si wafer substrate.
microstructure contained NWs that can affect the Raman spectra and cause phonon confinement. Moreover, the Raman peak at 522 cm 1 in the CdS NW sample corresponded to a single crystal Si substrate. Furthermore, the CdS necklace microstructure showed higher intensity compared with that of the CdS NW structure. In addition, the intensity ratio of the overtone of phonon to fundamental (I2LO/I1LO) of the NW structure was less than that of the necklace microstructure. These results indicate that the strength of the exciton–LO phonon coupling in necklaces is stronger than that of the other sample. 4. Conclusions
Fig. 5. PL spectra of CdS nanostructures prepared via thermal evaporation: (A) CdS necklace micro-structures on ITO-coated glass and (B) CdS NWs on Si wafer substrate.
3.3.3. Raman spectra Raman spectroscopy is a powerful tool for investigating the doping concentration, lattice defect identification, and crystal orientation of materials. However, it cannot be used to distinguish between the two structures because the frequencies of the cubic vibrational modes nearly matched those of the hexagonal modification [15,16]. Fig. 6 shows the Raman spectra of the CdS necklace microstructures prepared on ITO-coated glass and CdS NWs grown on Si wafer substrate. The appeared Raman peaks correspond to the first and second order longitudinal optical (LO) phonon modes, which are polarized in the x–z face with strong coupling to excitations along the c-axis [17]. The fundamental (1LO) and overtone (2LO) modes for the CdS necklace microstructure were located, respectively, at 301 and 601 cm 1, whereas those of the CdS NWs structures peaked at 301.5 and 603.5 cm 1. For bulk CdS, the fundamental (1LO) and overtone (2LO) modes were located at 305 and 605 cm 1, respectively [18]. Thus, the 1LO and overtone 2LO modes for the CdS necklace microstructure and NWs are redsifted. The redshift in the Raman peak positions could be due to the phonon confinement effect [19]. However, the CdS necklaces showed greater phonon confinement effect. The CdS necklace
CdS necklace microstructure was prepared on ITOcoated glass, whereas NWs structure was grown on Si(100) wafer substrates at 520 1C. The XRD analysis of the grown necklace-like microstructure and NWs showed that both structures were formed with hexagonal structure. The optical band gap values of the grown CdS necklace-like microstructures and NWs are found to be 2.44 eV and 2.59 eV, respectively. Two emission bands were observed in both spectra. The necklace structure emitted at 484 and 653 nm, whereas the NW emission was located at 478 and 557 nm. The green PL emission in both structures showed a strong blue-shift compared with the optical band gap of CdS. Raman spectra of CdS necklace and NWs structures showed that the fundamental (1LO) and overtone (2LO) modes were redshifted compared with Raman bands of bulk CdS. Furthermore, the intensity ratio of the overtone of phonon to fundamental (I2LO/I1LO) of the NW structure being less than that of the necklace microstructure indicates that the strength of the exciton–LO phonon coupling in necklaces is stronger than that of the other sample. References [1] M.A. Mahdi, Z. Hassan, S.S. Ng, J.J. Hassan, S.K. Mohd Bakhori, Thin Solid Films 520 (2012) 3477–3484. [2] M.A. Mahdi, J.J. Hassan, Naser M. Ahmed, S.S. Ng, Z. Hassan, Superlattices Microstruct. 54 (2013) 137–145. [3] M.A. Mahdi, J.J. Hassan, S.S. Ng, Z. Hassan, Naser M. Ahmed, Phys. E: Low-dimensional Syst. Nanostruct. 44 (2012) 1716–1721.
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[4] M.A. Mahdi, S.J. Kasim, J.J. Hassen, A.A. Swadi, S.K.J. Al-Ani, Int. J. Nanoelectron. Mater. 2 (2009) 163–172. [5] M.A. Mahdi, J.J. Hassan, Z. Hassan, S.S. Ng, J. Alloys Compd. 541 (2012) 227–233. [6] S. Guduru, V.P. Singh, S. Rajaputra, S. Mishra, R. Mangu, I.S. Omer, Thin Solid Films 518 (2010) 1809–1814. [7] M.A. Mahdi, J.J. Hassan, S.S. Ng, Z. Hassan, J. Cryst. Growth 359 (2012) 43–48. [8] S. Kar, S. Chaudhuri, J. Phys. Chem. B 110 (2006) 4542–4547. [9] G. Ghione, Semiconductor Devices for High-Speed OptoelectronicsCambridge University Press, UK, 2009. [10] B.D. Cullity, Elements of X-Ray Diffraction, Addison-Wesley, Reading, MA, USA, 1972. [11] A. Phuruangrat, T. Thongtem, S. Thongten, Characterization of multipod cadmium sulfide nanostructures synthesized by aminethermal method chalcogenide letters 7 (2010) 605–608.
[12] C.T. Tsai, D.S. Chuu, G.L. Chen, S.L. Yang, J. Appl. Phys. 79 (1996) 9105–9109. [13] R.N. Ahmad-Bitar, Renew. Energy 19 (2000) 579–586. [14] S. Celebi, A.k. Erdamar, A. Sennaroglu, A. kurt, H.Y. Acar, J. Phys. Chem. B 111 (2007) 12668–12675. [15] S. Yan, et al., J. Solid State Chem. 182 (2009) 2941–2945. [16] Z. Angel, et al., Solid State Commun. 104 (1997) 161–166. [17] T. Thongtem, A. Phuruangrat, S. Thongtem, Ceram. Int. 35 (2009) 2817–2822. [18] M. Abdulkhadar, B. Thomas, Nanostruct. Mater. 5 (1995) 289–298. [19] F.J. Owens, C.P. Poole, The Physics and Chemistry of Nanosolids, John Wiley & Sons, Inc., Hoboken, New Jersey, USA, 2008.
Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Optical properties of CdS micro/nanocrystalline structures prepared via a thermal evaporation method M.A. Mahdi a,n, J.J. Hassan a, S.J. Kasim a, S.S. Ng b, Z. Hassan b a
Basrah Nanomaterials Research Group (BNRG), Physics Department, College of Science, Basrah University, Basrah, Iraq Nano-Optoelectronics Research and Technology Laboratory (N.O.R.), School of Physics, University Sains Malaysia, 11800 Penang, Malaysia b
a r t i c l e i n f o
Keywords: CdS nanostructure Thermal evaporation Photoluminescence spectra
abstract Cadmium sulfide (CdS) micro/nanocrystalline structures were fabricated by thermal evaporation in the absence of a catalyst on ITO-coated glass and Si(100) substrates. The samples prepared at 520 1C showed two structures of CdS: nanowires (NWs) and necklace-like microstructures grown on Si(111) and ITO substrates, respectively. The crystalline structures of the grown CdS necklace-like microstructures and NWs were studied using XRD analysis. The CdS necklaces and NWs structures were formed with hexagonal (wurtzite) structure. Two emission bands (green and red) were observed in the photoluminescence spectra of the prepared samples. The green PL emission in both structures showed a strong blue-shift compared with the optical band gap of CdS. Raman spectra of the grown CdS micro/nanocrystalline structures show redshift in the fundamental (1LO) and overtone (2LO) peaks location compared with Raman bands of bulk CdS. & 2014 Elsevier Ltd. All rights reserved.
1. Introduction Nanomaterials have recently received much attention because of their unique physical, chemical, and mechanical properties. The control on the fundamental properties of materials, such as magnetic, optical, and electrical properties, can be achieved without altering their chemical composition by controlling the size, structure type, and surface states of the nanocrystalline materials [1]. Wide band gap II–VI semiconductors have been studied intensively for many years because of their great potential for a variety of applications, especially in optoelectronic devices [2,3]. Cadmium sulfide (CdS) is one of the most important II–VI semiconductors, with a direct band gap of 2.42 eV at room temperature [4,5]. CdS is extensively used in optoelectronic devices because it tunes emission in the visible-
n
Corresponding author. Tel.: þ964 778686096. E-mail address: [email protected] (M.A. Mahdi).
http://dx.doi.org/10.1016/j.mssp.2014.04.015 1369-8001/& 2014 Elsevier Ltd. All rights reserved.
light range; it is also widely used to fabricate hetrojunction solar cells and optical sensors. Various growth methods are used to fabricate CdS nanowires (NWs), including direct current electrodeposition [6], solvothermal synthesis [7], and thermal evaporation [8]. Among these methods, thermal evaporation is used to grow single crystalline CdS micro-rod and nanosheet for high speed photodetction devices [2,3]. Single crystalline semiconductors have received much attention in the fabrication of high-quality optoelectronic devices because of their unique optical and electrical properties [9]. In this work, CdS micro/nanocrystalline structures were fabricated by thermal evaporation. Surface morphology and crystalline structure as well as the optical properties of the prepared samples were investigated. 2. Experimental CdS micro/nanocrystalline structures were fabricated by thermal evaporation in the absence of a catalyst. In this work, indium tin oxide (ITO) glass (1.5 cm 2.5 cm) and
88
M.A. Mahdi et al. / Materials Science in Semiconductor Processing 26 (2014) 87–92
Si(1 0 0) were used as substrates. CdS powder (1 g; purity, 99.999%; Fluka Chemical Co.) was placed in an alumina boat and fixed at the center of a quartz tube in a furnace. The ITO glass substrate was fixed at the distance of 25 cm from the center. One end of the quartz tube was connected to a gas source that supplied 50 sccm of argon (Ar) gas as the transport gas. The furnace temperature was then raised gradually to 900 1C within 30 min at 30 1C/min. This temperature was maintained for 2 h. During the deposition, the ITO glass and Si(1 0 0) substrates temperature was 520 1C. Once the deposition was completed, the furnace was cooled to room temperature under Ar gas flow. The morphology of the prepared CdS micro/nanostructures was characterized using Scanning Electron Microscopy (SEM; model JSM-6460LV). The structural properties of the grown CdS micro/nanostructures were examined using X-ray diffraction (XRD) (X'Pert Pro MPD X-ray diffractometer, PANalytical Company) with CuKα radiation. The optical absorption of the prepared CdS nanocrystalline structures was conducted using a Shimadzu UV–vis spectrophotometer model UV-18s00. The samples were removed from the substrates and immersed in water to measure the optical absorption. Room temperature photoluminescence (PL) and Raman spectra were obtained by a Horiba Jobin Yvon HR 800 UV equipment which uses 325 nm, 20 mW HeCd and 514.5 nm, 20 mW Ar ion laser light for excitation, respectively. 3. Results and discussion 3.1. Surface morphology Fig. 1 shows the SEM images and EDS spectra of the CdS nanowires (NWs) prepared via thermal evaporation on ITO-coated glass and Si(1 0 0) substrates. The sample prepared at 520 1C showed two structures of CdS: NWs and necklace-like microstructures. The CdS NWs were grown via solid–vapor–solid (SVS) mechanism on the substrate surface. The grown NWs contain nodes along the wire length (shown in Fig. 1B), which served as growth centers for the necklace-like microstructures. Fig. 1C shows the CdS NWs grown on Si substrate at 520 1C. Unlike the CdS structures on the ITO–glass substrates, the NWs appeared homogenous and node-free. The effect of the substrate on NW morphology and shape starts from the initial stage of growth. The substrate type has an important function in CdS micro/nanostructures growth. The EDS results showed that the Cd/S ratios were 1.19 and 1.11 for the CdS necklaces grown on ITO–glass and CdS NWs prepared on Si wafer, respectively. Fig. 2 shows a schematic for the suggested growth mechanism of the CdS necklace microstructures. 3.2. Crystalline structure The crystalline structures of the grown CdS necklacelike microstructures and NWs were studied using XRD analysis. The CdS necklaces and NWs structures were formed with hexagonal (wurtzite) structure, as shown in the XRD patterns (Fig. 3A and B) compared with a standard XRD database (PDF-4, 00-0010780). The lattice constants a
and c for the hexagonal structures were calculated using the following equation [10]: ! 2 2 2 1 4 h þ hk þ k l ¼ ð1Þ þ 2; 2 2 3 a c ðdhkl Þ where dhkl is the interplanar spacing of the atomic planes, and h, k, and l are Miller indices. The prepared structures were 4.14 and 6.72 Å, corresponding to the standard data of the bulk CdS material. The preferred orientation (Phkl) of the prepared CdS nanostructures was determined using the flowing formula [11]: P ðhklÞ ¼
I ðhklÞ ∑I 0ðhklÞ I 0ðhklÞ ∑I ðhklÞ
;
ð2Þ
where I(hkl) and I0 (hkl) are the integrated intensities of the experimental and random (simulated) XRD diffractions, respectively. For a random orientation, Phkl equals one. For a particular (hkl) plane with a preferred orientation, Phkl is greater than one. The necklace structure had a P002 of 1.53 and P101 of 1.07. These results indicate that plane (002) is the preferred orientation of the CdS nanostructures. In addition, the preferred orientation of the CdS NWs was along the (002) plane, where the P002 value was 3.64 compared with the 0.77 for the (101) plane. 3.3. Optical properties 3.3.1. Optical absorption The optical absorption of CdS necklaces and CdS NWs was investigated at room temperature over the wavelength of 400–800 nm to evaluate the forbidden energy gap (Eg). Fig. 4A shows the ultraviolet–visible (UV–vis) absorption spectra of the CdS necklace-like microstructures and NWs, respectively. The CdS NWs showed blueshift of the absorption edge compared with the absorption edge of CdS necklaces which can be related to the quantum size effect of the CdS NWs. The optical band gap Eg of direct semiconductor can be estimated from the following relationship [1]: α ¼ Aðhν Eg Þ1=2 ;
ð3Þ
where A is a constant and hν is the energy of the illuminating photon. For the direct band gap, Eg, which can be calculated from the linear portion of the curve, is extrapolated to (αhυ)2 ¼ ¼0 when (αhυ)2 is plotted as a function of hυ. The optical band gap values of the grown CdS necklacelike microstructures and NWs estimated from Fig. 4B are 2.44 eV, and 2.59 eV, respectively. 3.3.2. Photoluminescence spectra Fig. 5A and B shows, respectively, the PL spectra of the CdS necklaces and NWs structures prepared via thermal evaporation. Two emission bands were observed in both spectra. The necklace structure emitted at 484 and 653 nm, whereas the NW emission was located at 478 and 557 nm. The green PL emission in both structures showed a strong blue-shift compared with the optical band gap of CdS (512 nm). This shift in the emission band may be attributed to the quantum size effect of the CdS
89
Intensity (a.u.)
M.A. Mahdi et al. / Materials Science in Semiconductor Processing 26 (2014) 87–92
Intensity (a.u.)
Energy (KeV)
Cd: 52.65% S: 47.35%
Energy (KeV) Fig. 1. SEM images and EDS spectra of CdS nanostructures prepared via thermal evaporation: (A, B) CdS necklace micro-structures on ITO-coated glass and (C) CdS nanowires on Si wafer substrate.
NWs and the surface states. Moreover, the main emission peak intensity of the necklaces and NW structures at 484 and 478 nm, respectively, was strong, indicating that electro-hole recombination occurred near the band edge. Furthermore, the emission band of the NWs was sharper than that of the necklace structure. These results may be attributed to the following reasons. First, the NW
dimension (diameter) was less than that of the necklace structure that contained CdS grains in micrometer scale. Second, the Cd/S ratio affected the emission band intensity and location. The Cd/S ratio for the necklace and NW was 1.11 and 1.19, respectively. Thus, the emission intensity of the NW structure was higher because of its shape and dimensions. The intensity ratio of the main emission to
90
M.A. Mahdi et al. / Materials Science in Semiconductor Processing 26 (2014) 87–92
Cd+S vapor
Cd+S atoms aggregator on the nodes
Formation of necklace micro-structure Fig. 2. Schematic of the suggested growth mechanism of the CdS necklace micro-structures.
Fig. 3. XRD patterns of CdS nanostructures prepared via thermal evaporation: (A) CdS necklace micro-structures on ITO-coated glass and (B) CdS NWs on Si wafer substrate.
that of defects peak (Imain/Idefect) of the necklace and NW structure was 2.5 and 5.9, respectively. The broad emission band of the necklace and NW around 653 and 557 nm (yellow band), respectively, can be associated with the structure defects or surface-trap emission. Excess Cd in CdS leads to the formation of an acceptor level within the CdS band gap. Thus, the yellow band may be attributed to the transition of the Cd-interstitial donors to the valence band [12]. However, Ahmad-Bitar [13] suggested more reasons for the appearance of the yellow emission band. He attributed this appearance to recombination via surface localized states, transition from ICd to the valence band, and transition from the interstitial Cd–Cd vacancy complex that acts as a donor to an accepter level. The intensity ratio of the band-edge emission to the defect emission (IBE/ID) was 2.46. In addition, the emission band in the red region
Fig. 4. (A) Optical absorption of CdS necklace micro-structures and CdS NWs and (B) plot of (αhν)2 vs. (hν) of CdS necklace micro-structures and CdS NWs.
(700–900 nm) disappeared, indicating the absence of sulfur vacancies that act as deep traps for electrons in the conduction band. This absence resulted in the red luminescence of CdS through the transfer to surface molecules or pre-existing trapped holes [14].
M.A. Mahdi et al. / Materials Science in Semiconductor Processing 26 (2014) 87–92
91
Fig. 6. Raman spectra of (a) CdS necklace micro-structures on ITO-coated glass and (b) CdS NWs on Si wafer substrate.
microstructure contained NWs that can affect the Raman spectra and cause phonon confinement. Moreover, the Raman peak at 522 cm 1 in the CdS NW sample corresponded to a single crystal Si substrate. Furthermore, the CdS necklace microstructure showed higher intensity compared with that of the CdS NW structure. In addition, the intensity ratio of the overtone of phonon to fundamental (I2LO/I1LO) of the NW structure was less than that of the necklace microstructure. These results indicate that the strength of the exciton–LO phonon coupling in necklaces is stronger than that of the other sample. 4. Conclusions
Fig. 5. PL spectra of CdS nanostructures prepared via thermal evaporation: (A) CdS necklace micro-structures on ITO-coated glass and (B) CdS NWs on Si wafer substrate.
3.3.3. Raman spectra Raman spectroscopy is a powerful tool for investigating the doping concentration, lattice defect identification, and crystal orientation of materials. However, it cannot be used to distinguish between the two structures because the frequencies of the cubic vibrational modes nearly matched those of the hexagonal modification [15,16]. Fig. 6 shows the Raman spectra of the CdS necklace microstructures prepared on ITO-coated glass and CdS NWs grown on Si wafer substrate. The appeared Raman peaks correspond to the first and second order longitudinal optical (LO) phonon modes, which are polarized in the x–z face with strong coupling to excitations along the c-axis [17]. The fundamental (1LO) and overtone (2LO) modes for the CdS necklace microstructure were located, respectively, at 301 and 601 cm 1, whereas those of the CdS NWs structures peaked at 301.5 and 603.5 cm 1. For bulk CdS, the fundamental (1LO) and overtone (2LO) modes were located at 305 and 605 cm 1, respectively [18]. Thus, the 1LO and overtone 2LO modes for the CdS necklace microstructure and NWs are redsifted. The redshift in the Raman peak positions could be due to the phonon confinement effect [19]. However, the CdS necklaces showed greater phonon confinement effect. The CdS necklace
CdS necklace microstructure was prepared on ITOcoated glass, whereas NWs structure was grown on Si(100) wafer substrates at 520 1C. The XRD analysis of the grown necklace-like microstructure and NWs showed that both structures were formed with hexagonal structure. The optical band gap values of the grown CdS necklace-like microstructures and NWs are found to be 2.44 eV and 2.59 eV, respectively. Two emission bands were observed in both spectra. The necklace structure emitted at 484 and 653 nm, whereas the NW emission was located at 478 and 557 nm. The green PL emission in both structures showed a strong blue-shift compared with the optical band gap of CdS. Raman spectra of CdS necklace and NWs structures showed that the fundamental (1LO) and overtone (2LO) modes were redshifted compared with Raman bands of bulk CdS. Furthermore, the intensity ratio of the overtone of phonon to fundamental (I2LO/I1LO) of the NW structure being less than that of the necklace microstructure indicates that the strength of the exciton–LO phonon coupling in necklaces is stronger than that of the other sample. References [1] M.A. Mahdi, Z. Hassan, S.S. Ng, J.J. Hassan, S.K. Mohd Bakhori, Thin Solid Films 520 (2012) 3477–3484. [2] M.A. Mahdi, J.J. Hassan, Naser M. Ahmed, S.S. Ng, Z. Hassan, Superlattices Microstruct. 54 (2013) 137–145. [3] M.A. Mahdi, J.J. Hassan, S.S. Ng, Z. Hassan, Naser M. Ahmed, Phys. E: Low-dimensional Syst. Nanostruct. 44 (2012) 1716–1721.
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[4] M.A. Mahdi, S.J. Kasim, J.J. Hassen, A.A. Swadi, S.K.J. Al-Ani, Int. J. Nanoelectron. Mater. 2 (2009) 163–172. [5] M.A. Mahdi, J.J. Hassan, Z. Hassan, S.S. Ng, J. Alloys Compd. 541 (2012) 227–233. [6] S. Guduru, V.P. Singh, S. Rajaputra, S. Mishra, R. Mangu, I.S. Omer, Thin Solid Films 518 (2010) 1809–1814. [7] M.A. Mahdi, J.J. Hassan, S.S. Ng, Z. Hassan, J. Cryst. Growth 359 (2012) 43–48. [8] S. Kar, S. Chaudhuri, J. Phys. Chem. B 110 (2006) 4542–4547. [9] G. Ghione, Semiconductor Devices for High-Speed OptoelectronicsCambridge University Press, UK, 2009. [10] B.D. Cullity, Elements of X-Ray Diffraction, Addison-Wesley, Reading, MA, USA, 1972. [11] A. Phuruangrat, T. Thongtem, S. Thongten, Characterization of multipod cadmium sulfide nanostructures synthesized by aminethermal method chalcogenide letters 7 (2010) 605–608.
[12] C.T. Tsai, D.S. Chuu, G.L. Chen, S.L. Yang, J. Appl. Phys. 79 (1996) 9105–9109. [13] R.N. Ahmad-Bitar, Renew. Energy 19 (2000) 579–586. [14] S. Celebi, A.k. Erdamar, A. Sennaroglu, A. kurt, H.Y. Acar, J. Phys. Chem. B 111 (2007) 12668–12675. [15] S. Yan, et al., J. Solid State Chem. 182 (2009) 2941–2945. [16] Z. Angel, et al., Solid State Commun. 104 (1997) 161–166. [17] T. Thongtem, A. Phuruangrat, S. Thongtem, Ceram. Int. 35 (2009) 2817–2822. [18] M. Abdulkhadar, B. Thomas, Nanostruct. Mater. 5 (1995) 289–298. [19] F.J. Owens, C.P. Poole, The Physics and Chemistry of Nanosolids, John Wiley & Sons, Inc., Hoboken, New Jersey, USA, 2008.