Reply to “Comments on Electric‐Field‐Assisted Growth of Highly Uniform and Oriented Gold Nanotriangles on Conducting Glass Substrates”

Size and shape control is a critical problem in the utility of nanomaterials as their physical and chemical properties depend strongly on structural parameters. Optical properties and catalysis of metal nanoparticles (NPs) reflect the structure-dependence most dramatically. Nanotriangles (NTs) or nanoprisms are a new class of nanomaterials and their properties such as near-infrared (NIR) absorption, anisotropic electrical conductivity and strong enhancement of electric fields at the vertices are expected to make them useful for a variety of applications in photonics and optoelectronics, information storage, optical sensing, biological labeling, imaging, metal enhanced fluorescence and surface-enhanced Raman scattering (SERS). Chemical, photochemical, and biological routes for their synthesis in solution are documented. Precise control of size and shape of NTs is still a major unsolved issue and post synthetic procedures are necessary to obtain reasonable monodispersity. For many sensing and catalytic applications in nanotechnology, one requires a programmed assembly of such nanostructures on planar surfaces. Periodic arrays of triangles can be made by nanosphere lithography. Several other strategies have been employed for the fabrication of NTs on planar surfaces such as seed mediation, sputter deposition and thermal vapor deposition. But ordered assembly is still a difficult task. In this report, we present an ‘electrical potential assisted’ method for the synthesis of uniform equilateral gold NTs, exhibiting preferred orientation having high near infrared absorption and strong surface enhanced Raman activity. The method used here for the preparation of NTs is a combination of the seed mediated growth process and the electrochemical deposition of metal particles on conducting glass surfaces. The indium tin oxide (ITO) coated conducting glass surfaces were cleaned and nanoparticles were immobilized on them through a self assembled monolayer. Afterwards, the potential assisted chemical growth was carried out as shown in the schematic (Fig. 1A) which resulted in highly uniform array of equilateral NTs on the ITO surface (see the Experimental Sec.). The gold NTs formation on ITO substrate was studied by optical absorption measurements, atomic force microscopy (AFM) and dynamic force microscopy (DFM). Figure 1B shows the time dependent UV-vis-NIR spectra of the formation of NTs. Growth of seeds on the surface was confirmed from the characteristic absorption in the UV-vis spectrum, showing only one peak with a maximum at 529 nm (trace A, Fig. 1B). As the growth proceeds, there was the emergence of a broad peak in the NIR region. Also observed was a distinct color change of the nanoparticle coated ITO glass from colorless to magenta purple over a period 1 h. The peak in the NIR region is attributed to in-plane surface plasmon resonance (SPR) of NTs supported on ITO and that at 540 nm is due to their out of plane surface plasmon resonance. The absorption centered in between 730–800 nm is attributed to the quadrupole resonance of the nanotriangles. Weak intensity of this peak may be due to the surface contact of the NTs. Earlier reports of NTs immobilized on substrates showed similar results. During the course of the NT growth, the intensity as well as the absorption maximum of the in-plane surface plasmon resonance increased. After a particular time interval, there was no apparent change observed in both the intensity as well as the absorption maximum which suggested that the NT growth stopped after 1 h. Even after 5 h of the growth process, there was no considerable change in the NIR absorption maximum compared to that after 1 h growth. Similar observations were also made by Mirkin et al. for the solution phase growth of NTs. The red shift in the absorption maxima compared to the earlier reports of NTs can be attributed to the increased edge length, larger particle density, increased surface contact and extended delocalization of the in-plane electrons. The slight red shift in the absorption maxima of the out of plane SPR is due to the smaller increase in the thickness of the NTs. A part of this intensity may also be due to spherical particles closer to the ITO surface, although not visible in AFM. From the AFM analysis it was confirmed that all the triangles observed are equilateral (Fig. 2A) and have flat surfaces. Figure 2B is a 3D image of the nanotriangles formed after 1 h of growth, showing uniform and ordered stacking of the NTs. C O M M U N IC A TI O N