Energy Focus: Cu-doping enables high-efficiency flexible-substrate CdTe solar cells
- PDF / 349,549 Bytes
- 2 Pages / 585 x 783 pts Page_size
- 107 Downloads / 208 Views
a
b
c
d
“Chemical patchiness” guides pH-dependent assembly of nonspherical nanostructures
UvdW (kBT) 0
ssembling the same type of nanoscale building blocks into different hierarchical structures can be difficult, requiring different surfactants or external forces during the self-assembly process. Researchers D.A. Walker, E.K. Leitsch, R.J. Nap, I. Szleifer, and B.A. Grzybowski from Northwestern University now demonstrate that it does not have to be this way; assembly can be as simple as adjusting the pH of an aqueous solution. In a recent study, published as the cover article of Nature Nanotechnology’s September issue (DOI: 10.1038/ NNANO.2013.158), this pH adjustment strategy was used to assemble gold/ silver nano-dumbbells into zippers or cross-stacked structures. The secret to this pH-dependent self-assembly lies in the non-uniform curvature and charging across the dumbbell-shaped particle. By surface functionalizing complexly shaped nanoparticles with 1-mercaptoundecanoic acid, the research group engineered “chemical patchiness”, that is, a non-uniform distribution of charge density, across the surface. The nano-dumbbell exhibits a lower curvature throughout the central region, which leads to a higher surface density of the surfactant molecules’ acidic head groups along the “handle” of the dumbbells. This region therefore exhibits a higher volumetric charge density than the ends of the nano-dumbbell. If the nano-dumbbells are dissolved in a solution with a pH > 9, electrostatic
Energy Focus Cu-doping enables highefficiency flexible-substrate CdTe solar cells
S
olar cells based on cadmium telluride (CdTe) currently occupy a market position second only to silicon-based devices, and have recently achieved significant efficiency improvements. However, their manufacturing costs must continue to deMRS BULLETIN
Twisting
–5
A
772
Offset
•
VOLUME 38 • OCTOBER 2013
•
50 nm
50 nm
50 nm
–10 –15 –20 –25
90°
90°
90° –30
Different nano-dumbbell assemblies arise from electrostatic patchiness. Transmission electron microscope (TEM) images and schemes show patchy nano-dumbbells assembled into (a) zippers, (b) intermediate zipper/cross-stacks, and (c) cross-stacks; schematics shown are topographic reconstructions based on TEM analysis (red regions denote the loci of highest negative charge); (d) van der Waals energies of different particle arrangements are shown. Reproduced with permission from Nature Nanotech. 8 (2013), DOI: 10.1038/NNANO.2013.158. © 2013 Macmillan Publishers Ltd.
repulsion from this higher charge density around their “handles” overrides van der Waals attractions between the particles, and a zipper-type assembly occurs. In contrast, a gradual reduction in pH results in a crossstack arrangement as the central regions of the dumbbells are now less charged and can approach each other more closely. Finite element calculations show that cross-stacking is the energetically most favorable form of assembly, but as the research team found out, it can only be achieved by pre-assembling the particle
Data Loading...
