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Colloidal aggregation revisited: New insights based on fractal structure and surface-enhanced raman scattering
A417 126
Surface Science 158 (1985) 126-146 North-Holland, Amsterdam
THE SHORT-RANGE
MECHANISM
SCATI'ERING
*
T.E. FURTAK
a n d D. R O Y
OF SURFACE ENHANCED
RAMAN
Department of Physics, Rensselaer Polytechnic Institute, Troy, New York 12181, USA Received 3 September 1984; accepted for publication 29 November 1984 In addition to the electromagnetic influences which increase the local field at the surface of suitably rough free-electron-like metals, molecule and site specific electronic resonances are also frequently operative. This leads to enhanced scattering beyond that caused by the electromagnetic mechanism alone. The electronic resonances involve charge-transfer excitation between members of an active complex MwAgxCly(H20) z which involves the probe molecule M and, in the electrochemical environment, the constituents of the solution. These complexes allow the resonance to be communicated to Raman transitions in all members of the complex. The stability of the complexes are sensitive to changes in the environment, such as the applied voltage and to the presence of foreign metal ions, through induced changes in y, the number of halide or pseudo-halide ions in the complex. Through analysis of the vibrational signature of the probe molecule under various environmental influences we are able to observe subtle changes in the character of the bonding between that molecule and the rest of the complex. These changes can also be interpreted as indirect results of changes in the value of y.
Surface Science 158 (1985) 147-164 North-Holland, Amsterdam
147
COLLOIDAL AGGREGATION REVISITED: NEW INSIGHTS BASED ON FRACTAL STRUCTURE AND SURFACE-ENHANCED RAMAN SCATTERING D.A. WEITZ and M.Y. LIN
Exxon Research and Engineering Co., Route 22 East, Annandale, New Jersey 08801, USA and C.J. S A N D R O F F
Bell Communications Research, 600 Mountain Avenue, Murray Hill, New Jersey 07974, USA Received 3 September 1984 We have examined both the structure and surface chemistry of gold clusters formed by the kinetic aggregation of colloidal gold particles. The highly disordered, ramified aggregates can be very accurately described as self-similar or fractal objects with a fractal dimension equal to 1.75. Spectroscopic studies performed with surface-enhanced Raman scattering (SERS), clearly indicate that colloidal gold surfaces are highly heterogeneous, consisting both of donor and acceptor sites which can be identified as Au(0) and Au(1), respectively. Aggregation occurs when negatively charged species are displaced from the gold surface by more strongly bound molecular adsorbates, with the rate determined by the nature and concentration of the displacing species. The new insights afforded by the fractal description of the structure of the aggregates and the SERS probe of the chemical nature of the colloid surface should lead to a more complete understanding of the basic mechanisms of colloid aggregation. This potential is illustrated with a quantitative description of the dynamics of aggregate growth measured by dynamic light scattering.
Surface Science 158 (1985) 126-146 North-Holland, Amsterdam
THE SHORT-RANGE
MECHANISM
SCATI'ERING
*
T.E. FURTAK
a n d D. R O Y
OF SURFACE ENHANCED
RAMAN
Department of Physics, Rensselaer Polytechnic Institute, Troy, New York 12181, USA Received 3 September 1984; accepted for publication 29 November 1984 In addition to the electromagnetic influences which increase the local field at the surface of suitably rough free-electron-like metals, molecule and site specific electronic resonances are also frequently operative. This leads to enhanced scattering beyond that caused by the electromagnetic mechanism alone. The electronic resonances involve charge-transfer excitation between members of an active complex MwAgxCly(H20) z which involves the probe molecule M and, in the electrochemical environment, the constituents of the solution. These complexes allow the resonance to be communicated to Raman transitions in all members of the complex. The stability of the complexes are sensitive to changes in the environment, such as the applied voltage and to the presence of foreign metal ions, through induced changes in y, the number of halide or pseudo-halide ions in the complex. Through analysis of the vibrational signature of the probe molecule under various environmental influences we are able to observe subtle changes in the character of the bonding between that molecule and the rest of the complex. These changes can also be interpreted as indirect results of changes in the value of y.
Surface Science 158 (1985) 147-164 North-Holland, Amsterdam
147
COLLOIDAL AGGREGATION REVISITED: NEW INSIGHTS BASED ON FRACTAL STRUCTURE AND SURFACE-ENHANCED RAMAN SCATTERING D.A. WEITZ and M.Y. LIN
Exxon Research and Engineering Co., Route 22 East, Annandale, New Jersey 08801, USA and C.J. S A N D R O F F
Bell Communications Research, 600 Mountain Avenue, Murray Hill, New Jersey 07974, USA Received 3 September 1984 We have examined both the structure and surface chemistry of gold clusters formed by the kinetic aggregation of colloidal gold particles. The highly disordered, ramified aggregates can be very accurately described as self-similar or fractal objects with a fractal dimension equal to 1.75. Spectroscopic studies performed with surface-enhanced Raman scattering (SERS), clearly indicate that colloidal gold surfaces are highly heterogeneous, consisting both of donor and acceptor sites which can be identified as Au(0) and Au(1), respectively. Aggregation occurs when negatively charged species are displaced from the gold surface by more strongly bound molecular adsorbates, with the rate determined by the nature and concentration of the displacing species. The new insights afforded by the fractal description of the structure of the aggregates and the SERS probe of the chemical nature of the colloid surface should lead to a more complete understanding of the basic mechanisms of colloid aggregation. This potential is illustrated with a quantitative description of the dynamics of aggregate growth measured by dynamic light scattering.