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This paper reports a rational design of branched titanium (Ti) nanorods formed by glancing angle physical vapor deposition and their applications as substrates for surface-enhanced Raman scattering (SERS). Ti nanorods with branches have larger surface areas than non-branched nanorods. However, Ti surface oxidizes easily resulting in very little SERS effect. The SERS sensitivity of the branched titanium nanorod is improved by annealing Ti nanorods in nitrogen in an effort to reduce oxidation. Additionally, the plasmonic resonance of the branched titanium nanorod is further improved by coating the top of the nanorods and branches with silver (Ag). The sensitivity of the SERS substrates is about 3700% that of as-deposited branched Ti nanorods with a native oxide layer. Our investigation provides a mechanism to fabricate sensitive SERS sensors of Ti nanorods that are known to be thermally and chemically stable and compatible with silicon-based electronics. [ABSTRACT FROM AUTHOR]

Nanorods grow in two possible modes during physical vapor deposition (PVD). In mode I, monolayer surface steps dictate the diameter of nanorods. In mode II, multiple-layer surface steps dictate the diameter, which is the smallest possible under physical vapor deposition [5,10]. This paper reports closed-form theories of terrace lengths and nanorod diameter during the growth in mode I, as a function of deposition conditions. The accompanying lattice kinetic Monte Carlo simulations verify these theories. This study reveals that (1) quasi-steady growth exists for each set of nanorod growth conditions, and (2) the characteristic length scales, including terrace lengths and nanorod diameter at the quasi-steady state, depend on the deposition conditions – deposition rate F , substrate temperature T , and incidence angle θ – only as a function of l 2 D /tan θ, with l 2 D = 2 ( v 2 D F cos θ ) 1 3 as a diffusion-limited length scale and v 2 D as the atomic diffusion jump rate over monolayer surface steps. [ABSTRACT FROM AUTHOR]

This letter proposes and experimentally demonstrates that oxygen, through action as a surfactant, enables the growth of aluminum nanorods using physical vapor deposition. Based on the mechanism through which oxygen acts, the authors show that the diameter of aluminum nanorods can be controlled from 50 to 500 nm by varying the amount of oxygen present, through modulating the vacuum level, and by varying the substrate temperature. When grown under medium vacuum, the nanorods are in the form of an aluminum metal - aluminum oxide core-shell. The thickness of the oxide shell is ~2 nm as grown and is stable when maintained in ambient for 30 days or annealed in air at 475 K for 1 day. As annealing temperature is increased, the nanorod morphology remains stable while the ratio of oxide shell to metallic core increases, resulting in a fully aluminum oxide nanorod at 1,475 K. [ABSTRACT FROM AUTHOR]

Ag nanorods may serve as sensors in the detection of trace amounts of chemical agents, even single molecules, through surface enhanced Raman spectroscopy (SERS). However, thermal coarsening of Ag nanorods near room temperature limits their applications. This letter proposes the use of a thin oxide capping layer to enhance the thermal stability of Ag nanorods beyond 100 °C. Using electron microscopy characterization and SERS tests, the authors show that the proposed method is effective in stabilizing both morphology and sensitivity of Ag nanorods. The results of this work extend the applicability of Ag nanorods as chemical sensors to higher temperatures. [ABSTRACT FROM AUTHOR]

The growth of crystalline nanorods has become a common practice in the absence of a solid framework, in either theoretical or conceptual form. This article presents such a framework and puts it in historical perspective of a broader field of crystal growth. This framework derives from three scientific advancements in crystal growth, with focus on multiple-layer surface steps: (I) the diffusion barrier of adatoms down multiple-layer surface steps, (II) the formation and stability of multiple-layer surface steps, and (III) the dimension of surface facets that are bounded by competing monolayer and multiple-layer surface steps. While this framework has only a partial foundation of theoretical formulation, it is more complete conceptually. As an example of impact, this framework predicts that growth of Al nanorods is not feasible using physical vapor deposition at ambient conditions; this prediction has not been proven wrong by any available experiments. [ABSTRACT FROM AUTHOR]

One dimensional titanium nanorod structures formed by glancing angle physical vapor deposition have branches while other hexagonal closed packed metals do not. Based on physical vapor deposition and characterizations using electron microscopy and X-ray diffraction, this paper reports that Ti nanorod branching occurs at a low homologous temperature of 0.28. The side surface of the nanorods consists of { 10 1 ¯ 1 } facets arranged in a zigzag shape. Further, branches form on the { 10 1 ¯ 1 } side facets that are parallel to the deposition flux. The length of the branches increases as they are farther away from the nanorod top and tend to reach a constant. The top surface facet of Ti nanorods is {0001} and that of the branches is { 10 1 ¯ 1 }. The insight into conditions for branching, together with the determination of the morphology and crystal orientation of the branches, lay the foundation for further studies of branching mechanisms and driving force. [ABSTRACT FROM AUTHOR]

In catalysis applications, it is desirable to have nanorods of smaller diameter. However, if the separation of nanorods is smaller than their diameter, nanorods will merge to become dense films. The separation of nanorods derives from the separation of nucleating clusters on the substrate. This letter reports a theory of the smallest separation of nanorods. This theory is in closed form and is verified by accompanying lattice kinetic Monte Carlo simulations. The theoretical prediction of transition from nanorods to dense films-when the separation of nanorods is smaller than their diameter-is in good agreement with in-house experimental observation. [ABSTRACT FROM AUTHOR]

This paper proposes a mechanism of controlling the diameter and separation of metallic nanorods from physical vapor deposition through self-organized seeds and experimentally demonstrates the feasibility using Ag as the prototype metal, In as the seed, and Si the substrate. Being non-wetting on Si substrates, deposited In atoms self-organize into islands. Subsequently deposited Ag atoms attach to In islands, rather than to Si substrates, due to preferential bonding and geometrical shadowing. The experimental results show that self-organized In seeds of 5 nm nominal thickness give rise to the best separation and the smallest diameter of Ag nanorods. [ABSTRACT FROM AUTHOR]