Controlling electrical conduction through noble metal thin films by surface plasmon resonance

The collective oscillations of surface charges (surface plasmons) induced by light-matter interactions were predicted in the 1950s to influence electrical conduction in 2D noble metals. Primarily two mechanisms were predicted and later by Frohlich which could affect the electron transport in metals; (1) Umklapp electron-electron scattering and (2) attractive interaction between free electrons in transition metals because of screening of the d-band electrons by the s-band electrons. [1] However, there is no experimental evidence to date on how such oscillations might influence the electrical conductivity of 2D metals. We have conducted in-situ measurements of the surface plasmons and electrical resistivity of thin film gold. We observe striking correlations between the electrical resistivity of noble metal thin films and surface plasmon resonance

Since their realization in the 1950s, there has been substantial interest in the physics of collective charge oscillations. The ability to control electrical conduction in thin film devices (for example, in field-effect transistors) by surface plasmon resonance (SPR) offers far-reaching consequences for several different types of application, for example, telecommunications, electronics, and sensors. Bohm and Pines made pioneering contributions to the field in the 1950s [1][2][3][4]. Since their early development, plasmon oscillations haves been of immense interest to the condensed matter physics community, particularly volume plasmons, and the phenomenon is now well understood.
When energetic electrons pass through a metallic foil, they experience a characteristic energy loss due to the excitation of plasmons in the sea of conduction electrons [5]. The free-electron approximation model of metals, in which high-density free electrons ( ) where n is the density of the free electrons, e is the magnitude of the electronic charge, and e m is the mass of an electron. The plasmon frequency is related to the frequency-dependent dielectric function of the material through For free-electron metals, the energies of the volume plasmons are rather high; they range from approximately 8 eV for Li to about 16 eV for Al [6].
Surface plasmon excitations, on the other hand, are low-energy surface charge density oscillations. The frequency of these high-energy excitations (usually generated by using optical frequencies) is given by where 2 the two-dimensional density of the occupied surface states [7][8][9][10][11].
Continued interest in surface plasmons has led to the discovery of various fundamental physical processes and applications. For example, (i) the strong electromagnetic fields associated with surface plasmon excitations have contributed to the development of surfaceenhanced Raman scattering [12] and (ii) the high sensitivity of SPR to differences in the dielectric properties across thin metal-film/dielectric interfaces has led to the development of numerous sensors [13].
The most commonly utilized nonradiative surface plasmons, excited by optical frequencies, exhibit q ω ∝ dispersion. On the other hand, low-energy acoustic surface plasmons, which can be excited by long-wavelength infrared sources, have a linear dispersion ( q ω ∝ ). Their energies are just about right to influence the electronic properties of materials. The possibility that these low-energy acoustic plasmons can influence physical processes, like the electrical conductivity of metals, was discussed by Bohm and Pines in the 1950s [1][2][3][4]. Pines thought that for the electrical conductivity to be affected, the current had to be changed through electron-electron collisions in two ways: by Umklapp electron-electron scattering or, for highly anisotropic electron energy surfaces, by collisions between electrons of different effective masses. Pines also refers to the possibility of the scattering of s-electrons by d-electrons in transition metals [4]. The overlap between the partially filled d-and s-band electrons in transition metals plays an important role in controlling electron conduction in two dimensional noble metals. [14,15] Since then, the possibility has been studied by others. For example, Garland considered a two-band model consisting of the s-states for the nearly free electrons and d-states for the tightly bound electrons [16]. In 1968, Frohlich proposed an alternate mechanism for electron pairing through acoustic plasmons in transition metals [17].   [18][19][20][21], this system utilizes the recently developed fixeddetector Kretschmann configuration optical system for SPR measurements. One of the advantages of the fixed-detector system is that it does not require the use of a ( , 2 ) goniometer [22]. The angular scans were accomplished by optically steering the incident laser beam. For the electrical resistivity measurements, four contact points on the film were established using a Signatone four-probe assembly. This assembly was mounted on a precision micrometer to allow displacement of the contact pins along the normal to the surface of the film. For the four-probe current and voltage measurements, a source meter (Keithley, SMU-2450) was used to apply constant currents to the outer pins of the fourprobe assembly and a voltmeter (Keithley, 2182A) was used to record the voltages between the inner two pins of the four-probe unit.
For each angle of incidence, a total of 100 current versus voltage measurements were made for three different current ranges: (1) low current range, from 10 nA to 1 μA, (2) mid current range, 1 μA to 10 μA, and (3) high current range, 10 μA to 100 μA.