January_February_2022_AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | J A N U A R Y / F E B R U A R Y 2 0 2 2 2 4 R esearchers at North Carolina State University recently discov- ered the impact of single and double helix screw dislocations on the crystal growth and mechanical proper- ties of solid state materials. Screw dislo- cations play a critical role in the growth of crystalline structures by providing a continuous source of surface steps, which represent available sites for crys- tal growth. However, one perplexing observation is that experimentally de- rived Burgers vectors are found to be far larger than the elementary Burg- ers vectors derived from crystal struc- tures. This mystery has been solved with the discovery of double helix screw dislocations. Two parallel straight dislocations have a repulsive interaction and can- not pair up to increase the magnitude of the effective Burgers vector. Howev- er, an attractive interaction can develop if these screw dislocations become he- lical. This author’s research shows that pure screw dislocations can become helical by absorbing defects such as va- cancies and develop attractive interac- tion with another helical dislocation to form a double helix screw dislocation. These single and double helix screw dislocations can also combine to cre- ate even larger effective Burgers vectors and lead to the formation of interest- ing nanostructures with a large Eshel- by twist. These large Burgers vectors also unravel the formation mechanism for nanopipes and micropipes with hollow cores as well as nanotubes with Eshel- by twists in technologically important materials. These include materials such as SiC, GaN, AlN, and ZnO required for a variety of advanced solid state devic- es including advanced light emitting di- odes, nanosensors, andhigh-frequency/ high-power devices. Further, advance- ments in understanding how nano- structures form are expected to lead to additional applications in every- thing from electronics and informa- tion technology to healthcare and the life sciences. RECENT RESEARCH Screw dislocations with Burgers vectors parallel to dislocation lines play a critical role in the crystal growth and mechanical properties of solid state materials [1,2] . Creation of a screw dis- location can be visualized by cutting a crystal along the plane and then dis- placing one face with respect to the other by a lattice (Burgers) vector. The boundary of this cut represents a screw dislocation along which lattice planes are arranged in a helical way [1] . This he- lical structure can provide a continu- ous source of surface steps for crystal growth along the axes of screw disloca- tions. This concept, originally proposed by Sir Charles Frank, solved a long- standing puzzle in materials science and explained crystal growth at super- saturations as low as 1% [3,4] . Without these dislocations, crys- tal growth from the vapor phase re- quired supersaturations exceeding 1.5. The crystal growth velocity is direct- ly proportional to the fraction of avail- able sites for attachment, which can be provided by the steps associated with emerging screw dislocations [5] . This self-seeding mechanism of crys- tal growth is applicable across the scale ranging from whiskers to nano- wires [6-8] . Moreover, it can overcome the use of catalysts in the growth of nano- structures and minimize impurity con- tamination. Catalysts often lead to undesirable impurity contamination, where a few atoms can lead to large local concentrations and changes in properties. However, dislocations are non- equilibrium defects. Their presence raises the system energy, which is pro- portional to the square of the mag- nitude of the Burgers vector. With DOUBLE HELIX DISCOVERY IMPACTS MATERIALS SYNTHESIS FROM NANOSCALE TO MESOSCALE Pure screw dislocations can become helical by absorbing defects such as vacancies and develop attractive interaction with another helical dislocation to form a double helix structure. J. Narayan, FASM,* North Carolina State University, Raleigh *Member of ASM International