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\section{Gold Free synthesis}  Despite the advancement of Au catalyzed synthesis of GaAs NW, Au free synthesis techniques are being investigated. Studies reveal that Au incorporation leads to scattering centers that disrupt desirable optical properties, and the Au catalyst itself is too expensive for industry scale up of synthesis. \cite{1966} Due to these issues an Au free synthesis is desirable. Current techniques range from "Ga-assisted growth", selective area epitaxy (SAE), solution based growth, and oxide assisted growth.\cite{Chatillon_1995}\cite{Yu_2003}\cite{Shi_2001}  Oxide assisted growth works similarly to VLS but instead of an Au catalyst, and oxide species in conjunction with laser ablation is necessary to catalyze NW growth. GaAs NW growth via this method has been reported, with the oxide species being $Ga_{2}O_{3}$ and growth being maintained in the same direction as VLS.\cite{Duan_2000}\cite{Shi_2001} The process begins with a quartz tube containing a Si surface mounted in a high-temperature tube furnace containing 25\% gallium oxide and 75\% GaAs powder. The tube is evacuated, heated to $930^{\circ}C$, then a laser ablation (10 Hz, 400 mJ/pulse, \lambda $\lambda$  = 248nm) takes place for 5h. Nanowires with 10-120nm diameters resulted. "Gallium assisted growth" is also similar to VLS (Ga playing the role of Au).\cite{Morral_2011} The growth condition are chosen by again analyzing a phase diagram to create a situation in which Ga liquid is in equilibrium with solid GaAs (this temperature is called the "congruent temperature" and was found to be $630^{\circ}C$)\cite{Chatillon_1995}. Arsenic adatoms (atoms "on top of" the crystal surface plane) diffuse into the Ga liquid droplet to form the GaAs NW, and these adatoms can be deposited via MBE, or MOVPE.