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Calendar: ALS Calendar
Title: ALS Special Seminar | Ron Grimm
When: 09/ 9/2013 1:30 PM - 2:30 PM
Description: ALS Special Seminar

Monday, September 9, 2013
​6-2202 ​Conf Rm
1:30 - 2:30 pm

​Zahid Hussain

Presented by:
Ron Grimm
Cal Tech

Methyl-terminated silicon microwire arrays for solar fuels applications. Surface structure, stability, and performance.

Silicon microwire arrays demonstrate promising light capture and energy conversion properties for photovoltaic and solar fuels applications. However, it remains unclear whether the passivation strategies applied to bulk, Si(111) systems will apply to these highly faceted, and more complex systems. A two-step chlorination-methylation wet process generates a highly ordered, methyl-terminated surface on Si(111) that exhibits excellent oxidative and electronic stability, and this work interrogates whether a two-step passivation strategy passivates surfaces relevant to Si microwire array-based PV devices.

Here, a two step chlorination-methylation process functionalized the surfaces of both hydrogen-terminated silicon microwire arrays as well as bulk wafers of the Si(211) and Si(110) orientations observed on the faceted microwires. Surface science studies including XPS, IR, and LEED quantified the surface passivation, while microwave photoconductivity and photoelectrochemistry interrogated the interfacial electronic properties.

The surface science results demonstrated that a two step chlorination-methylation process produces methyl-terminated surfaces on silicon microwires, planar Si(211), and planar Si(110). In contrast to methylated Si(111) with trace amounts of surface oxidation, all three methyl-terminated surfaces studied exhibited 5-10% surface oxidation following a 3-day exposure to air-ambient, but little additional oxidation with prolonged air exposures. Methyl-terminated Si(211) and Si(110) maintained sufficient long-range structure to exhibit LEED patterns comparable to their freshly hydrogen-terminated counterparts.

Although the methyl-terminated surfaces exhibited surface oxidation, photoelectrochemistry and microwave conductivity experiments confirmed that the surfaces exhibited good electronic structure with a low density of defect states and good open circuit photovoltages in non-aqueous photoelectrochemical systems. These results suggest that methyl-terminated microwires are sufficiently stable against the formation of silicon oxide subsurface back bonds that contribute to deleterious surface recombination. These results also indicate the promising potential for mixed methyl/catalyst monolayers on silicon microwire arrays for solar fuels applications.
Location: 6-2202 Conf Room