Jijin Yang

Rapid and precise scanning helium ion microscope milling of solid-state nanopores for biomolecule detection

We report the formation of solid-state nanopores using a scanning helium ion microscope. The fabrication process offers the advantage of high sample throughput along with fine control over nanopore dimensions, producing single pores with diameters below 4 nm. Electronic noise associated with ion transport through the resultant pores is found to be comparable with levels measured on devices made with the established technique of transmission electron microscope milling. We demonstrate the utility of our nanopores for biomolecular analysis by measuring the passage of double-strand DNA.

Iron meteorite evidence for early formation and catastrophic disruption of protoplanets

In our Solar System, the planets formed by collisional growth from smaller bodies. Planetesimals collided to form Moon-to-Mars-sized protoplanets in the inner Solar System in 0.1–1 Myr, and these collided more energetically to form planets. Insights into the timing and nature of collisions during planetary accretion can be gained from meteorite studies. In particular, iron meteorites offer the best constraints on early stages of planetary accretion because most are remnants of the oldest bodies, which accreted and melted in <1.5 Myr, forming silicate mantles and iron-nickel metallic cores. Cooling rates for various groups of iron meteorites suggest that if the irons cooled isothermally in the cores of differentiated bodies, as conventionally assumed, these bodies were 5–200 km in diameter. This picture is incompatible, however, with the diverse cooling rates observed within certain groups, most notably the IVA group, but the large uncertainties associated with the measurements do not preclude it. Here we report cooling rates for group IVA iron meteorites that range from 100 to 6,000 K Myr-1, increasing with decreasing bulk Ni. Improvements in the cooling rate model, smaller error bars, and new data from an independent cooling rate indicator show that the conventional interpretation is no longer viable. Our results require that the IVA meteorites cooled in a 300-km-diameter metallic body that lacked an insulating mantle. This body probably formed ∼4,500 Myr ago in a ‘hit-and-run’ collision between Moon-to-Mars-sized protoplanets. This demonstrates that protoplanets of ∼103 km size accreted within the first 1.5 Myr, as proposed by theory, and that fragments of these bodies survived as asteroids.

Direct and Transmission Milling of Suspended Silicon Nitride Membranes With a Focused Helium Ion Beam

Helium ion milling of suspended silicon nitride thin films is explored. Milled squares patterned by scanning helium ion microscope are subsequently investigated by atomic force microscopy and the relation between ion dose and milling depth is measured for both the direct (side of ion incidence) and transmission (side opposite to ion incidence) regimes. We find that direct-milling depth varies linearly with beam dose while transmission-milling depth varies with the square of the beam dose, resulting in a straightforward method of controlling local film thickness.

Helium ion microscopy: a new tool for imaging novel mesoporous silica and organosilica materials

Helium ion microscopy (HIM) has been used to image mesoporous silica and organosilica for the first time. Images of chiral nematic silica, ethylenesilica, and new benzenesilica reveal the structural organization, pore dimensions and connectivity of these materials on the nanometer length scale.

Ultrastructural analysis of wild type and mutant Drosophila melanogaster using helium ion microscopy

Insects have evolved numerous adaptations to survive a variety of environmental conditions. Given that the primary interface between insects and the environment is mediated through their skin or cuticle, many of these adaptations are found in extraordinary cuticle diversity both in morphology and structure. Not all of these adaptions manifest themselves in changes in the chemical composition of the cuticle but rather as elaborations of the surface structures of the cuticle. Typically the examination of these micro- and nanoscale structures has been performed using scanning electron microscopy (SEM). Typically, in order to decrease surface charging and increase resolution, an obscuring conductive layer is applied to the sample surface, but this layer limits the ability to identify nanoscale surface structures. In this paper we use a new technology, helium ion microscopy (HIM) to examine surface structures on the cuticle of wild type and mutant Drosophila. Helium ion microscopy permits high resolution imaging of biological samples without the need for coating. We compare HIM to traditional SEM and demonstrate certain advantages of this type of microscopy, with our focus being high resolution characterization of nanostructures on the cuticle of Drosophila melanogaster and potentially other biological specimens.

Structural and magnetic properties of biaxially textured NiFe2O4 thin films grown on c-plane sapphire

Chemical solution deposition is used to grow biaxially textured NiFe2O4 (NFO) thin films on (0001) sapphire substrates; a high degree of out-of-plane orientation in the 〈111〉 direction is confirmed by θ–2θ X-ray diffraction and pole figures. X-ray φ-scanning indicates in-plane texture and an epitaxial relationship between NFO (111) and Al2O3 (0001) in two crystallographic variants. The out-of-plane magnetization exhibits improved Mr/Ms from 0.5 in 110 nm-thick films to 0.8 in 60 nm-thick films. Compared to uniaxially textured NFO films on platinized silicon, the out-of-plane coercivity is reduced by 20%. The improved out-of-plane magnetic anisotropy is comparable to epitaxial NFO films of similar thickness deposited by pulsed laser deposition and sputtering.