We have grown CdSe semiconductor films on glass substrates and the films were coated with Au nanoparticles of 10 nm in size by the pulsed-laser deposition technique. The films demonstrate a large enhancement of Raman intensity and photoluminescence of CdSe semiconductor via excitation of surface plasmon resonances in proximate gold metal nanoparticles deposited on the surface of CdSe film. These observations suggest a variety of approaches for improving the performance of devices such as photodetectors, photovoltaics, and related devices, including biosensors.
©2008 Optical Society of America
Recently, surface plasmon resonances in metallic nanoparticles are being explored for a variety of applications including powerful and evolving toolkit for biological detection [1,2] and molecular sensing [3,4], focusing of light , subwavelength photonics , and near-field optical microscopy . The phenomena of surface plasmon excitations arise from the large electromagnetic field enhancement near the metal nanoparticle’s surface depending on the resonance wavelength on its size, shape, and local dielectric environment. The incident light generates the plasmon excitation of the metallic nanoparticle, which involves the light-induced motion of the valence electrons. Hence, the cross section for the elastic light scattering from the metal nanoparticle can be million-fold larger than the cross section for absorption or emission of electromagnetic radiation from any molecule or nanocrystal. The excitation of surface plasmons in metal nanoparticles placed onto a semiconductor might be expected to enhance optical phenomena, such as optical absorption and photoluminescence (PL) of incident photons within the semiconductor region near each nanoparticle due to localized field amplification.
A variety of approaches for increasing optical absorption in semiconductors based on excitation of surface plasmon resonance in metal nanoparticles, which are in contact with the semiconductors, has been proposed. Recent studies on spherical Au nanoparticles deposited on Si p-n junction photodiodes increase the absorption of light over a broad spectral range via the interaction of the incident electromagnetic radiation with surface plasmon resonance in the nanoparticles . Similarly, enhancement in short-circuit current density and energy conversion efficiency in engineered amorphous silicon p-i-n solar cells is achieved via improved transmission of electromagnetic radiation arising from forward scattering of surface plasmon polariton modes in Au nanoparticles deposited above the amorphous silicon film . On the other hand, the distribution of metal nanoparticles can be used for absorbing or scattering the solar spectrum that may be relevant for a variety of energy harvesting or conserving applications . Among other applications, the surface enhanced Raman spectra (SERS) have also been reported using semiconductor nano-structures, such as diamond nanocrystals using Au or Ag as SERS-active agent , and GaN nanocolumnar structures . Therefore, there is enough scope to exploit the metal nanoparticles for design of various functional devices for enhanced performance.
Apart from other chemical and physical deposition techniques, nanoparticle materials can be tailored by laser-induced particle aggregation and deposition [13–15]. It has recently been demonstrated that the pulsed laser can control and manipulate the particle size and shape [16,17] as well as pattern the materials by ablation, deposition, or etching . Here we show the enhancement of Raman intensity, PL, and absorption in CdSe semiconductor via excitation of surface plasmon resonances in Au nanoparticles insitu pulsed-laser deposited (PLD) on CdSe semiconductor film surface. We have demonstrated a large enhancement of Raman intensity, and photoluminescence of CdSe semiconductor films grown on glass substrates via excitation of surface plasmon resonances in proximate gold metal nanoparticles deposited on the surface of CdSe film.
CdSe films were grown by the multi-target UHV-PLD technique (KrF excimer, λ=248 nm, 20 ns pulse) with a pulse energy density of 1–1.5 J/cm2. The substrate was loaded to the chamber using load-lock facility attached to the chamber, and heated in the chamber just after the ultimate base pressure <2×10-8 Torr was reached. CdSe films were grown at 270 °C on glass substrates at ultra-high vacuum condition. CdSe single crystal target was used for the deposition. Au film on CdSe surface was deposited insitu from a high-purity Au target (99.999%) at 100 °C for better adherence. The number of laser pulses was optimized in order to obtain the desired thickness of the film. The X-ray diffraction (XRD) studies were carried out in a Rigaku X-ray diffractometer with CuKα radiation. Atomic force microscopic (AFM) images were taken using a Veeco Nanoscope-III. The optical properties of the films were characterized by micro-Raman scattering using Jobin-Yvon (LabRam) Raman spectrometer using He-Ne laser.
3. Results and discussion
Figure 1(a) presents the representative atomic force microscopic (AFM) image of Au(20nm)/CdSe(40nm)/glass film, showing dense gold nanoparticles. The nanoparticles have average size of 10 nm. 3-dimensional (3D) image of the surface of the film is shown in Fig. 1 (b) for clarity. The gold nanoparticles are very uniform in size. On the other hand, CdSe film grown on the glass shows smooth surface morphology.
Figure 2 presents the field-emission scanning electron microscopic image (FE-SEM) image of the Au(10nm)/CdSe(40nm)/glass film in order to confirm the spherical shape of the Au nanoparticles. It is very clear from the FE-SEM image that the Au nanparticles are, in fact, spherical in shape as seen from the AFM image. The size of the nanoparticles nearly 10 nm in diameter and these nanoparticles are fairly uniform in shape. However, some of the larger particles are believed due to the clustering of Au nanoparticles.
X-ray diffraction (XRD) measurements were performed in order to characterize the crystal structure of the CdSe and Au/CdSe films on glass substrates and shown in Fig. 3. The CdSe film displays a polycrystalline hexagonal wurtzite structure. However, the film is, some how, textured along  direction. The broad hump is due to the amorphous glass substrate for CdSe/glass films. The Au/CdSe films on glass substrates show pronounced Au(111) orientations, illustrating texturing of Au nanoparticles in  direction. The intense Au(111) Bragg peak for Au/CdSe films on glass can be attributed to the better texturing of the Au nanoparticles in  direction. The XRD and microscopic data indicate that the Au/CdSe/glass films are good in quality. Crystallite size of the Au nanoparticles was calculated from the line broadening the full width half maxima (FWHM or Γ) using Scherrer’s equation, only taking into account the attribution of line width to particle size, assuming the spherical shape of the particle size, which is, in fact, observed from the microscopic studies as described above. The equation for determining the crystallite size is Ds=k×~λ/Γ cosθ, where k is a constant (=0.89), λ is the X-ray wavelength, Γ is X-ray line width (FWHM) in radian, and θ is the Bragg angle for the diffracted X-ray. The XRD intensity for  direction is considered for the calculation. The XRD lines are shown in the inset of Fig. 3 for both 10 and 20 nm thick films. The calculated crystallite size is about 8 and 12 nm, respectively, for 10 and 20 nm thick films. The slight difference in crystallite size between two films is due to the clustering in later films.
Typical Raman scattering spectra of CdSe and Au/CdSe films are shown in Fig. 4. The peak at 210 cm-1 is due to the longitudinal optical phonon mode (LO) of wurtzite crystal structure of CdSe . The phonon frequency of the longitudinal optical 2LO mode of CdSe at was found at 419 cm-1. However, the central result of the Raman studies is the enhancement of the Raman intensity due to the plasmonic resonance of Au nanoparticle deposited on the top of CdSe films. An enhancement of Raman intensity of LO mode at 210 cm-1 was observed in CdSe/glass films as shown in Fig. 4. However, the enhancement factor of Raman intensity was found to be 3.2 with thickness of Au layer of 10 nm and increases to 5.3 with increase of thickness to 20 nm. Similar enhancement was observed for the 2LO mode located at 419 cm-1, but lower in magnitude compared to the LO mode. The increase in Raman intensity with increasing Au cluster size on CdSe can be attributed to the increase of density of Au nanoparticles.
Figure 5 shows the PL spectra of CdSe and Au(20nm)/CdSe on glass substrates at room temperature. The intensity of PL spectra of Au(20 nm)/CdSe films is stronger than that of CdSe/glass. The Au(20nm)/CdSe/glass has the strongest intensity. The enhancement factor of 9 was obtained for the light intensity at 720 nm in Au(20 nm)/CdSe on glass compared to CdSe/glass. The large PL enhancement is due to the localized enhancement of semiconductor optical absorption via excitation of surface plasmon resonances in proximate metal nanoparticles.
The absorption was increased with increasing Au layers, especially towards near infrared region. The electrons in the metal nanoparticle influences the surface plasmon absorption band of the metal nanoparticles, which enhance the absorption process of the semiconductor in contact with the metal nanoparticles as discussed above. The enhanced absorption process with increasing Au layer from 10 to 20 nm is attributed to the increase in density of Au nanoparticles or clusters that influences the optical phenomena due to enhanced plasmonic process. The emission from the Au nanostructures is enhanced by the surface plasmon resonances, which occur in gold nanostructures [20–22]. However, the emission characteristics follow from two mechanisms as described below.
The surface plasmons can increase the emission intensity through two mechanisms, (a) enhancement of radiative electron-hole recombination by local fields [21,23] and (b) nonradiative electron-hole recombination emitting surface plasmons which give rise to luminescence . However, these explanations do not explain the enhancement of the PL due to the lack of correlations among nanoparticle size and surface plasmon resonance wavelength.
The plasmon resonance of each individual metal nanoparticle is highly sensitive to small changes in the local refractive index, hence the local dielectric environment . The coupling of dielectric and electromagnetic behavior in the optical regime in nanostructured metals and semiconductors is likely the reason for enhanced optical response. This will enable and extend a variety of new and emerging approaches to the engineering of photonic and optoelectronic devices.
In summary, we have demonstrated the experimental evidence of large enhancement Raman intensity and photoluminescence via excitation of surface plasmon resonance in proximate metal nanoparticles deposited on the surface of CdSe semiconductor. Our results suggest that high performance of photodetectors, related optoelectronic, such as efficiency in thin-film solar cells, optical communication and sensing devices, including bio and molecular sensors, can be envisioned with improved functionality.
The work was supported by the NSF CREST, and the NASA URC grant. The authors are thankful to Carl Bonner for useful discussions.
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