Properties of gold nanostructures sputtered on glass
© Siegel et al; licensee Springer. 2011
Received: 26 May 2010
Accepted: 19 January 2011
Published: 19 January 2011
We studied the electrical and optical properties, density, and crystalline structure of Au nanostructures prepared by direct current sputtering on glass. We measured temperature dependence of sheet resistance and current-voltage characteristics and also performed scanning electron microscopy [SEM] analysis of gold nanolayers. It was shown that within the wide range of temperatures, gold nanolayers (<10 nm) exhibit both metal and semiconducting-like type of conductivity. UV/Vis analysis proved the semiconducting characteristic of intrinsic Au clusters. SEM analysis showed the initiatory stadium of gold layer formation to be running over isolated islands. Gold density calculated from the weight and effective thickness of the layers is an increasing function of the layer thickness up to approximately 100 nm. In thin layers deposited on solid surface, a lattice expansion is observed, which is manifested in the increase of the lattice parameter and the decrease of metal density. With increasing layer thickness, the lattice parameter and the density approach the bulk values.
Nanocrystalline thin solid films nowadays present enormous scientific interest, mainly due to their attractive novel properties for technological applications [1, 2]. The most important prerequisite for the preparation of high-quality film is an understanding of its growth dynamics and structure in different phases of deposition.
In the course of the twentieth century, the theory of size-dependent effects in metal thin layers was further developed by numerous scientists, and various approaches to the problem were proposed. For isolated metal particles' behavior at exiguous dimensions (1D and 2D), quantum size effects are decisive, whereas for ultrathin metal layers both surface effects and quantum size effects must be considered [3, 4]. These phenomena can be attributed to a high nanolayer and/or nanoparticle surface-to-bulk ratio. Hand in hand with the reduction of nanoparticle dimension, surface atoms' proportion increases dramatically; thus, commonly known physical properties of the bulk materials change, e.g., density and melting point of Au nanoparticle decreases [5–7]. Properties of metal layers are affected by electron scattering on phonons, on imperfections, and at layer boundaries. While the first two types of scattering occur also in bulk metal, the last one plays a role only in thin layers, and it is responsible for the reduction of the electric conductivity of thin layers . Mathematical formula for the calculation of relaxation times for more than one scattering mechanism is given by Matthiessen's rule .
Gold is known as a shiny, yellow noble metal that does not tarnish, has a face-centered cubic structure, is non-magnetic, melts at 1,336 K, and has density a 19.320 g cm-3. However, a small sample of the same gold is quite different, providing it is tiny enough: 10-nm particles absorb green light and thus appear red. The melting temperature decreases dramatically as the sample size goes down . Moreover, gold ceases to be noble, and 2- to 3-nm nanoparticles are excellent catalysts which also exhibit considerable magnetism [4, 10]. At this size, Au nanoparticles also turn into insulators. Gold in the form of thin films is nowadays used in a vast range of applications such as microelectromechanical and nanoelectromechanical systems [11, 12], sensors , electronic textiles , bioengineering , generator of nonlinear optical properties , or devices for surface-enhanced Raman scattering .
The optical and electrical properties of Au nanoparticles have been studied on samples prepared by atom sputtering deposition approach onto porous alumina in . The electrical resistance measurement shows that the nanoparticles are conductive even at a small metal volume fraction. Due to the aggregation effect, the optical transmission spectra exhibited an enhanced transmition band around 500 nm arising from the surface plasmon resonance . Many authors have developed theories of distortion of crystalline lattice in nanostructures, some of them being applicable on nanoparticles. Spherical nanoparticles surrounded 'by air' have different behaviors as nanostructures deposited on solid surface. While in spherical nanoparticles a dominant effect is a lattice compression [9, 19–21], in other nanostructured materials (e.g., nanowires, nanolayers), a lattice expansion is observed [22, 23]. The compression can be explained by the Young-Laplace equation for spherical particles and the effect of decreasing size and a curvature of surface. The expansion on the other hand can be due to imperfections of the lattice and the size surface effects on nanostructures. More important is the effect of lattice imperfections which, on the other hand, may lead to a density decrease.
In this work, we studied the electrical and optical properties, density, and crystalline structure of Au nanostructures prepared by sputtering on glass. Measurement of the sheet resistance of gold nanostructures at room and low (LN2) temperatures proved the metal or semiconductive-like characteristic of the structures. Scanning electron microscopy [SEM] analysis showed the gold layer growth to be running over isolated islands. The mechanism of charge transfer and the optical excitation of metal particles were determined by measuring the electrical sheet resistance and UV/Vis spectrometry, respectively. The UV/Vis spectra were interpreted in the frame of the well-known Tauc's model , and the optical band gap (E g opt.) of ultrathin Au structures was calculated as a function of structure thickness. X-ray diffraction [XRD] analysis provided information about the crystalline structure and the lattice parameter values. Density of Au was calculated from the weight (gravimetry) and the effective thickness of Au layers which were measured by atomic force microscopy [AFM].
Substrate and Au deposition
The gold structures were sputtered on a 2 × 2-cm microscopic glass substrate, 1 mm thick, supplied by Glassbel Ltd., Czech Republic. Glass surface roughness of R a = 0.34 nm was measured at ""square 1.5 μm2. The sputtering was accomplished on a Balzers SCD 050 device from gold target (purity 99.99%, supplied by Goodfellow Ltd., Cambridge, UK). One slide was prepared during each sputtering operation. Deposition chamber was not equipped with a rotated sample holder. Under analogous experimental conditions, homogenous layers with uniform thickness were prepared . The deposition conditions were the following: direct current Ar plasma, gas purity 99.995%, discharge power of 7.5 W, Ar flow approximately 0.3 l s-1, pressure of 5 Pa, electrode distance of 50 mm, electrode area of 48 cm2, and reaction chamber volume approximately 1,000 cm3. The sputtering times vary from 4 to 500 s.
Metal structure thickness for chosen sputtering times (effective thickness) was examined using AFM. The AFM images were taken under ambient conditions on a Digital Instruments CP II setup. The samples, 1 cm2 in area, were mounted on stubs using a double-sided adhesive. A large area scanner was used, allowing an area up to 100 μm2 to be imaged. A Veeco phosphorus-doped silicon probe CONT20A-CP with spring constant 0.9 N m-1 was chosen. In the present experiment, structure homogeneity was tested by a scratch technique at ten different positions. The thickness of the structures was determined from the AFM scan done in contact mode . Thickness variations do not exceed 5%. All scans were acquired at a scanning rate of 1 Hz.
The electrical properties of gold structures were examined by measuring the electrical sheet resistance (R s). R s was determined by a standard two-point technique using a KEITHLEY 487 picoampermeter. For this measurement, additional Au contacts, about 50 nm thick, were created by sputtering. The electrical measurements were performed at a pressure of about 10 Pa to minimize the influence of atmospheric humidity. The temperature dependence of R s was determined on the samples placed in a cryostat evacuated to the pressure of 10-4 Pa. The samples were first cooled to the LN2 temperature and then gradually heated to room temperature. Typical error of the sheet resistance measurement did not exceed ± 5%.
The current-voltage [CV] characteristics were measured using picoampermeter KEITHLEY 487 (sheet resistance, >105 Ω) and multimeter UNI-T (sheet resistance, <105 Ω). The temperature dependence of CV characteristics was also determined. In that case, measured samples were placed into the cryostat at the temperature of liquid nitrogen and were gradually heated to room temperature.
XRD analysis was performed by an automatic powder refractometer Panalytical X'Pert PRO using a copper X-ray lamp (λ CuKα1 = 0.1540598 nm) equipped with an ultrafast semiconductor detector PIXcel. Measurement has passed on a symmetric Bragg-Brentano geometry. Diffractograms were registered in the angular range 2ϑ = (10° to 85°). Lattice parameter a of the cubic face-centered lattice of Au was calculated from diffraction lines location and its intensity using Rietveld's method. The lattice parameter could only be determined for samples with an Au thickness exceeding 10 nm.
UV/Vis spectra were measured using a Shimadzu 3600 UV-Vis-NIR spectrometer (Kyoto, Japan) in the spectral range from 200 to 2,700 nm. Evaluation of the optical spectra was performed using Film Wizard software with the aim of determining plasma frequency. Measured spectra were also interpreted in the frame of Tauc's model  using Tauc's equation α(ν) = A(hν - E g opt) x /hν, where α is the absorption coefficient of the substance, E g opt is the substance optical band gap, x is the parameter that gives the type of electron transition, and factor A depends on the transition probability and can be assumed to be constant within the optical frequency range . Optical band gap width, E g opt, of layers was assessed from the linear part of plot ((α(ν)⋅hν) x vs. hν). Indirect transition cannot be excluded in these layers, and therefore, x = 1/2 was used in the calculation.
Mettler Toledo UMX2 microbalance (Greifensee, Switzerland) was used for gravimetric determination of an amount of sputtered gold on a glass template. Density of Au layers was then calculated from the weight and effective layer thickness determined from the AFM scan.
Direct measurement of the layer thickness was accomplished by a SEM (JSM-7500F). The specimen for SEM examination was prepared by cross-sectioning of the metal-glass sandwich on a standard cross-section polisher, with focused ion beam (6-kV acceleration voltage).
Results and discussion
Thickness and morphology of Au structures
Electrical properties of Au structures
From the measurements of sheet resistance and CV characteristics result the semiconductor-like characteristic of Au at specific structure conditions (thickness, temperature). The observed semiconductor-like characteristic (decreasing resistance with increasing temperature, nonlinearity of CV characteristic) of ultrathin Au structures may originate from two undistinguishable phenomena. The first one results from a tunneling effect which occurs at discontinuous structures during resistance measurements . The second one originates from the semiconductor characteristic of the intrinsic cluster itself, which occurs in metal nanostructures of sufficiently small proportions . With respect to the experimental method used, it is impossible to distinguish which phenomenon prevails in prepared structures and contribute to the observed semiconductor-like behavior of Au nanostructures.
In order to investigate whether the intrinsic Au clusters forming ultrathin Au coverage exhibit semiconductor behavior, indeed we accomplished additional optical UV/Vis analysis.
Optical properties of Au structures
The UV/Vis spectra were also interpreted in the frame of Tauc's model  (see also above) and the optical band gap (E g opt.) calculated as a function of the structure thickness. The E g opt. as a function of the structure thickness is shown in Figure 7B. A non-zero value of E g opt. was detected in the case of Au structure thicknesses ranging from 2 to 30 nm, which corresponds with the sputtering times between 4 and 150 s. Apart from electrical measurements, optical methods do not require any conductive path between separated clusters during measurement. That is why optical-based methods are able to separate the contribution of tunneling effects to the properties of Au nanostructures, which cannot be omitted during electrical measurements of discontinuous metal layers. Optically analyzed evolution of band gap thus unambiguously confirms the semiconductive characteristic of intrinsic clusters forming Au nanolayers. However, even after the electrically continuous layer is formed (sputtering time of approximately 50 s, which corresponds to a structure thickness of approximately 10 nm), which is characterized by the creation of a conductive path between isolated clusters and a rapid decline of sheet resistance (see Figures 1 and 3), there still must exist regions of separated Au clusters in deposited layer which contribute to non-zero E g opt. up to the structure thickness of approximately 30 nm (see Figure 7B).
Lattice parameter and density of Au structures
With the aim of finding how the decline of lattice parameter influences the density of gold structures, we measured the effective thickness and the mass of deposited structures and calculated the effective density in a standard way. In Figure 8, the dependence of the density on the layer thickness is shown. The density increases with increasing layer thickness, and for about a 90-nm-thick layer, it achieves the density of bulk gold. The reduced density of thinner structures is probably due to the higher fraction of free volume in gold nanoclusters. As the gold clusters become greater , the free volume fraction decreases and the gold density gradually increases. It was reported earlier  that gold layers with thicknesses above 100 nm prepared on glass substrate exhibit quite a uniform density, with a mean value of 19.3 g cm-3 typical of bulk material. Theoretical Au density was calculated from the value of lattice parameter .
We observe a linear dependence between the sputtering time and the structure thickness even in the initial stadium of the Au growth. After the stage of nucleation, the growth of Au clusters proceeds mainly in the lateral direction. A rapid decline of the sheet resistance of the gold layer with increasing structure thickness indicates a transition from the discontinuous to the continuous gold layer. From the dependence of the sheet resistance on the sample temperature and from the measured CV characteristics of Au structures, it follows that the gold layers thicker than 10 nm exhibit a metallic characteristic. Structures with thicknesses between 5 and 10 nm exhibit a semiconductor-like characteristic at low temperatures and metalloid conductivity at higher temperatures. Layers with thicknesses below 5 nm exhibit semiconductive-like properties in the whole investigated temperature range. Optical absorption of the structures at the initial phase of the layer growth is a function of the gold cluster density. Plasma frequency (concentration of free carrier) increases with the layer thickness. UV/Vis analysis proved the semiconducting characteristic of intrinsic Au clusters. XRD measurements proved the monotonous decline of the lattice parameter with increasing structure thickness. Measurements of the effective thickness and weight of deposited structures showed that the Au density is an increasing function of structure thickness. For the layer thicknesses above 90 nm, the layer density achieves the bulk value.
This work was supported by the Grant Agency of the CR under the projects 106/09/0125 and 108/10/1106, Ministry of Education of the CR under Research program LC 06041, and Academy of Sciences of the CR under the projects KAN400480701 and KAN200100801. It was also founded by financial support from specific university research (MSMT no. 21/2010).
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