Richard G. Hobbs

Resolution Limits of Electron-Beam Lithography toward the Atomic Scale

We investigated electron-beam lithography with an aberration-corrected scanning transmission electron microscope. We achieved 2 nm isolated feature size and 5 nm half-pitch in hydrogen silsesquioxane resist. We also analyzed the resolution limits of this technique by measuring the point-spread function at 200 keV. Furthermore, we measured the energy loss in the resist using electron-energy-loss spectroscopy.

PEGylated gold nanoparticles: polymer quantification as a function of PEG lengths and nanoparticle dimensions

Au nanoparticles with diameters ranging between 15 and 170 nm have been synthesised in aqueous solution using a seed-mediated growth method, employing hydroxylamine hydrochloride as a reducing agent. Thiolated polyethylene glycol (mPEG-SH) polymers, with molecular weights ranging from 2100 to 51000 g mol−1, were used as efficient particle stabilising ligands. Dynamic light scattering and zeta potential measurements confirmed that the overall mean diameter and zeta potential of the capped nanoparticles increased in a non-linear way with increasing molecular weight of the mPEG-SH ligand. Electron microscopy and thermal gravimetric analysis of the polymer-capped nanoparticles, with a mean gold core diameter of 15 nm, revealed that the grafting density of the mPEG-SH ligands decreased from 3.93 to 0.31 PEG nm−2 as the molecular weight of the ligands increased from 2100 to 51400 g mol−1 respectively, due to increased steric hindrance and polymer conformational entropy with increase in the PEG chain length. Additionally, the number of bound mPEG-SH ligands, with a molecular weight of 10800 g mol−1, was found to increase in a non-linear way from 278 (σ = 42) to approximately 12960 PEG (σ = 1227) when the mean Au core diameter increased from 15 to 115 nm respectively. However, the grafting density of mPEG10000-SH ligands was higher on 15 nm Au nanoparticles and decreased slightly from 1.57 to 0.8 PEG nm−2 when the diameter increased; this effect can be attributed to the fact that smaller particles offer higher surface curvature, therefore allowing increased polymer loading per nm2. Au nanoparticles were also shown to interact with CT-26 cells without causing noticeable toxicity.

Seedless growth of sub-10 nm germanium nanowires.

We report the self-seeded growth of highly crystalline Ge nanowires, with a mean diameter as small as 6 nm without the need for a metal catalyst. The nanowires, synthesized using the purpose-built precursor hexakis(trimethylsilyl)digermane, exhibit high aspect ratios (>1000) while maintaining a uniform core diameter along their length. Additionally, the nanowires are encased in an amorphous shell of material derived from the precursor, which acts to passivate their surfaces and isolates the Ge seed particles from which the nanowires grow. The diameter of the nanowires was found to depend on the synthesis temperature employed. Specifically, there is a linear relationship between the inverse radius of the nanowires and the synthesis temperature, which can be explained by a model for the size-dependent melting of simple metals.

Determining the Resolution Limits of Electron-Beam Lithography: Direct Measurement of the Point-Spread Function

One challenge existing since the invention of electron-beam lithography (EBL) is understanding the exposure mechanisms that limit the resolution of EBL. To overcome this challenge, we need to understand the spatial distribution of energy density deposited in the resist, that is, the point-spread function (PSF). During EBL exposure, the processes of electron scattering, phonon, photon, plasmon, and electron emission in the resist are combined, which complicates the analysis of the EBL PSF. Here, we show the measurement of delocalized energy transfer in EBL exposure by using chromatic aberration-corrected energy-filtered transmission electron microscopy (EFTEM) at the sub-10 nm scale. We have defined the role of spot size, electron scattering, secondary electrons, and volume plasmons in the lithographic PSF by performing EFTEM, momentum-resolved electron energy loss spectroscopy (EELS), sub-10 nm EBL, and Monte Carlo simulations. We expect that these results will enable alternative ways to improve the resolution limit of EBL. Furthermore, our approach to study the resolution limits of EBL may be applied to other lithographic techniques where electrons also play a key role in resist exposure, such as ion-beam-, X-ray-, and extreme-ultraviolet lithography.

AXSIS: Exploring the frontiers in attosecond X-ray science, imaging and spectroscopy

X-ray crystallography is one of the main methods to determine atomic-resolution 3D images of the whole spectrum of molecules ranging from small inorganic clusters to large protein complexes consisting of hundred-thousands of atoms that constitute the macromolecular machinery of life. Life is not static, and unravelling the structure and dynamics of the most important reactions in chemistry and biology is essential to uncover their mechanism. Many of these reactions, including photosynthesis which drives our biosphere, are light induced and occur on ultrafast timescales. These have been studied with high time resolution primarily by optical spectroscopy, enabled by ultrafast laser technology, but they reduce the vast complexity of the process to a few reaction coordinates. In the AXSIS project at CFEL in Hamburg, funded by the European Research Council, we develop the new method of attosecond serial X-ray crystallography and spectroscopy, to give a full description of ultrafast processes atomically resolved in real space and on the electronic energy landscape, from co-measurement of X-ray and optical spectra, and X-ray diffraction. This technique will revolutionize our understanding of structure and function at the atomic and molecular level and thereby unravel fundamental processes in chemistry and biology like energy conversion processes. For that purpose, we develop a compact, fully coherent, THz-driven atto-second X-ray source based on coherent inverse Compton scattering off a free-electron crystal, to outrun radiation damage effects due to the necessary high X-ray irradiance required to acquire diffraction signals. This highly synergistic project starts from a completely clean slate rather than conforming to the specifications of a large free-electron laser (FEL) user facility, to optimize the entire instrumentation towards fundamental measurements of the mechanism of light absorption and excitation energy transfer. A multidisciplinary team formed by laser-, accelerator,- X-ray scientists as well as spectroscopists and biochemists optimizes X-ray pulse parameters, in tandem with sample delivery, crystal size, and advanced X-ray detectors. Ultimately, the new capability, attosecond serial X-ray crystallography and spectroscopy, will be applied to one of the most important problems in structural biology, which is to elucidate the dynamics of light reactions, electron transfer and protein structure in photosynthesis.

High-yield, ultrafast, surface plasmon-enhanced, Au nanorod optical field electron emitter arrays

In this work we demonstrate the design, fabrication and characterization of ultrafast, surface-plasmon enhanced Au nanorod photofield emitter arrays. We present a quantitative analysis of charge yield from plasmonic Au nanorod arrays fabricated by high-resolution electron beam lithography and triggered by 35 fs pulses of 800 nm light. We have accurately modeled both the optical field enhancement of Au nanorods in high-density arrays, and electron emission from those nanorods. We have considered the effects of surface plasmon damping induced by metallic interface layers at the substrate/nanorod interface on electron emission. We have identified the peak optical field at which the electron emission mechanism transitions from a 3-photon absorption mechanism to strong-field tunneling emission. Moreover, we have investigated the effects of nanorod array density on nanorod charge yield, including measurement of space-charge effects.

High-density Au nanorod optical field-emitter arrays

We demonstrate the design, fabrication, characterization, and operation of high-density arrays of Au nanorod electron emitters, fabricated by high-resolution electron beam lithography, and excited by ultrafast femtosecond near-infrared radiation. Electron emission characteristic of multiphoton absorption has been observed at low laser fluence, as indicated by the power-law scaling of emission current with applied optical power. The onset of space-charge-limited current and strong optical field emission has been investigated so as to determine the mechanism of electron emission at high incident laser fluence. Laser-induced structural damage has been observed at applied optical fields above 5 GV m(-1), and energy spectra of emitted electrons have been measured using an electron time-of-flight spectrometer.

High-energy surface and volume plasmons in nanopatterned sub-10 nm aluminum nanostructures

In this work, we use electron energy-loss spectroscopy to map the complete plasmonic spectrum of aluminum nanodisks with diameters ranging from 3 to 120 nm fabricated by high-resolution electron-beam lithography. Our nanopatterning approach allows us to produce localized surface plasmon resonances across a wide spectral range spanning 2-8 eV. Electromagnetic simulations using the finite element method support the existence of dipolar, quadrupolar, and hexapolar surface plasmon modes as well as centrosymmetric breathing modes depending on the location of the electron-beam excitation. In addition, we have developed an approach using nanolithography that is capable of meV control over the energy and attosecond control over the lifetime of volume plasmons in these nanodisks. The precise measurement of volume plasmon lifetime may also provide an opportunity to probe and control the DC electrical conductivity of highly confined metallic nanostructures. Lastly, we show the strong influence of the nanodisk boundary in determining both the energy and lifetime of surface plasmons and volume plasmons locally across individual aluminum nanodisks, and we have compared these observations to similar effects produced by scaling the nanodisk diameter.

Directed self-assembly of PS-b-PMMA block copolymer using HSQ lines for translational alignment

We report here the graphoepitaxial alignment of a lamellar forming PS-b-PMMA block copolymer (BCP) for directed self-assembly using topographical patterns (simple line structures) of hydrogen silsesquioxane (HSQ). The system demonstrates the importance of the sidewall chemistry on translational alignment of BCP domains. A method was developed where a silicon substrate was precoated with a hydroxyl-terminated random copolymer brush of PS-r-PMMA prior to the HSQ feature formation process. The brush ordains the vertical (to the substrate plane) alignment of the BCP lamellar microdomains. Translational BCP alignment is a result of PMMA selectively wetting the HSQ. The formed BCP pattern was selectively etched to remove the PMMA domain allowing direct imaging and to demonstrate capability in forming an on-chip mask.

Single step synthesis of Ge–SiOx core-shell heterostructured nanowires

The ability to control the morphology of heterostructured nanowires, through the design of precursors and by careful manipulation of reaction parameters, is important if these materials are to be utilised as components in future nanodevices. Traditionally core-shell heterostructured nanowires have been synthesised through multi-precursor and/or multi-step synthetic routes. In this paper we discuss the synthesis of core-shell, Ge–SiOx heterostructured nanowires using a single source precursor and a single step supercritical fluid reaction process.