Delivered by Ingenta to: Stevens Institute of Technology IP : 155.246.152.20 Wed, 05 Jan 2011 15:37:17 Copyright © 2010 American Scientific Publishers All rights reserved Printed in the United States of America Nanoscience and Nanotechnology Letters Vol. 2, 133–138, 2010 Manipulation of Low-Dimensional Nanomaterials Using Water Meniscus O. Sul, Chen-en Tsai, Ning Gao, and E. H. Yang ∗ Stevens Institute of Technology, Castle Point on Hudson, Hoboken, NJ 07030, USA This paper reports on a colloidal manipulation of low-dimensional nanomaterials such as nanopar- ticles and nanowires. This colloidal manipulation enables the arrangement of nanoparticles and nanowires on a substrate with controlled location and orientation. The conducted research includes the study of conditions such as the water contact angle by surface treatment and the nanowire length for the optimized colloidal manipulation of nanomaterials. The measured minimum water con- tact angle, required to drive gold nanoparticles, is 50  on top of a Teflon-coated surface. Further, the required maximum length of the nanowires to be driven by the meniscus overcoming friction between the nanowires and the surface is on the order of 1 micron. Keywords: Colloidal Assembly, Nanowire, Nanoparticle, Pulsed Laser Deposition, Water Contact Angle. 1. INTRODUCTION The assembly of individual nano-objects is critical for developing high-throughput nanofabrication techniques. There have been a number of reports on the assembly of individual or many nanoparticles, 1–5 and on nanowires 6–14 on a substrate in desired patterns. Among the assembly methods reported, the colloidal assembly of nanoparticles provides high assembly yields (∼90%) and controllability of individual objects. 1 2 4 A water meniscus surface can drive nanoparticles without losing them along its move- ment direction. Kraus 1 used this technique to assemble nanoparticles onto the pre-patterned, dimpled network of silicon or polymer substrates. In this technique, the water contact against the substrate is the critical factor in deter- mining the success of the colloidal assembly. If the water contact angle is relatively small below a threshold value, the meniscus does not have enough driving force on the nanoparticles. As a result, the nanoparticles are left behind the meniscus in the convective assembly. 4 However, if the angle is larger than the threshold, the nanoparticles fol- low the meniscus until they are trapped at the geometrical depressions, i.e., colloidal assembly. The natural questions following such an achievement are then: Can the same technique be used for the assembly of nanowires? What would the determining factors be for successful colloidal assembly? This paper presents our preliminary study on the optimized conditions for driving of nanoparticles and ∗ Author to whom correspondence should be addressed. nanowires using a water meniscus, which can potentially be used to assemble nanowires in a desired pattern (Fig. 1). It outlines the driving conditions of nanowires with water meniscus that are affected by the change in water con- tact angle after surface treatment, as well as the effect of nanowire friction. 2. COLLOIDAL ASSEMBLY OF NANOPARTICLES AND NANOWIRES 2.1. Nanoparticle Trapping Using Nano-Dimples If an object is immersed in a water drop on a surface, the object is trapped inside, and it cannot escape from the water–air interface. Even if the water–air interface moves due to evaporation, the object will follow the water move- ment to stay within the interface. This phenomenon is due to the water induced surface tension applied on the object. When a portion of the object is out of a water meniscus, the water surface tension induces a force along the water- object edge. Here, the circumferential length of a portion of the nanoparticle or a nanowire in the air is given by 2R for a particle where R is the radius and 2L + R for a wire where L is length and R is cross-sectional radius. For high aspect ratio nanowires, this can be simply approximated as 2 L. The meniscus driving force acting on a nanoparticle or a nanowire is given as (Fig. 2(a)): F m = Exposure length ×  × Sin (1) Nanosci. Nanotechnol. Lett. 2010, Vol. 2, No. 2 1941-4900/2010/2/133/006 doi:10.1166/nnl.2010.1070 133