Resonant Inelastic X-ray Scattering (RIXS)
 is an X-ray in, X-ray out technique that enables one to study the 
dispersion of excitations in solids. In this thesis, we investigated how
 various elementary excitations of transition metal oxides show up in 
RIXS spectra.

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Resonant Inelastic X-ray Scattering Studies of Elementary Excitations

In recent years, SPM-based lithography has attracted great attention because of its simplicity and precise control of the structure and location Many SPM lithography techniques based on mechanical scratching,1 anodization of Si surfaces,2 electrochemical decomposition of self-assembled monolayers,3 electric field-induced chemical reactions,4 electrochemical reactions in solution using electrochemical STM tips5 have been developed in the past decade Comprehensive reviews of SPM-related lithography can be found in the literature6 More recently, a “dippen” nanolithography (DPN) method has been invented that uses an atomic force microscope (AFM) tip as a “nib” to directly deliver organic molecules onto suitable substrate surfaces, such as Au7 By using this technique, organic monolayers can be directly written on the surface with no additional steps, and multiple inks can be used to write different molecules on the same surface However, the current “dip-pen” method can only be used to deliver organic molecules to the surface The long-term stability of the created structures is a potential problem Here we report a new electrochemical “dip-pen” lithography technique that can be used to directly fabricate metal and semiconductor nanostructures on surfaces This technique has all the advantages of the previous “dip-pen” technique and improves the thermal stability and chemical diversity of the structures because they now could be made of various inorganic materials Furthermore, the ability to directly fabricate metal or semiconductor nanostructures on surfaces with a high degree of control over location and geometry is of significant interest in nanotechnology Potentially, one could use this method to fabricate nanodevices with multiple metal and semiconductor components When AFM is used in air to image a surface, the narrow gap between the tip and surface behaves as a tiny capillary that condenses water from the air This tiny water meniscus is actually an important factor that has limited the resolution of AFM in air “Dip-pen” AFM lithography uses the water meniscus to transport organic molecules from tip to surface7 In our new technique, we also use the tiny water meniscus on the AFM tip as the transfer medium However, unlike in the previous AFM “dip-pen” method where water is only used as a solvent for the molecules, we have used this tiny water meniscus as a nanometer-sized electrochemical cell in which metal salts can be dissolved, reduced into metals electrochemically, and deposited on the surface (Figure 1) Although electric field-induced chemical reactions,4 electrochemical reactions in solutions using electrochemical STM,5 and electrochemical deposition using self-assembled monolayer as resist3c have been previously used to create metallic nanostructures, our method is the first that combines the versatility of electrochemistry with the simplicity and power of the DPN method to produce nanostructures with high resolution Electrochemical STM-based methods require that the substrates be metallic, but substrates used in our method do not have to be metallic since the control feedback of the AFM does not rely on the current between the tip and surface Si wafers coated with native oxide provides enough conductivity for the reduction of the precursor ions This development significantly expands the scope of DPN lithography, making it a more general nanofabrication technique that not only can be used to deliver organic molecules to surfaces but is also capable of fabricating metallic and semiconducting structures with precise control over location and geometry Because of the electrochemical nature of this new approach, we call this technique electrochemical “dip-pen” nanolithography (E-DPN) We have investigated the deposition of several metals and semiconductors on Si surfaces at room temperature using the E-DPN technique Here we show the deposition of Pt metal as an example8 The experiments were performed using a Nanoscape IIIa AFM (Digital Instruments) In a typical experiment, an ultrasharp silicon cantilever coated with H2PtCl6 is scanned on a cleaned P-type Si (100) surface with a positive DC bias applied on the tip During this lithographic process, H2PtCl6 dissolved in the water meniscus is electrochemically reduced from Pt(IV) to Pt(0) metal at the cathodic silicon surface and deposits as Pt nanofeatures according to the following equation:

Electrochemical AFM "dip-pen" nanolithography.

This paper describes the first example of the patterning of surfaces via nanocontact printing with chemically distinct features in the <50 nm scale over 3 × 3 mm2. These ultrasmall features are achieved via a combination of “sharp” and “hard” poly(dimethylsiloxane) stamps, keeping the contact area in the 50 nm domain, and high molecular weight inks to avoid diffusion. The patterns consist of dendrimers or proteins and can be used as a scaffold for further modification with gold nanoparticles.

Nanocontact Printing: A Route to Sub-50-nm-Scale Chemical and Biological Patterning

We describe a novel parallel method for the patterning of proteins with nanoscale resolution. Combining nanoimprint lithography (NIL) and molecular assembly patterning by lift-off (MAPL), we produc...

A Novel Approach to Produce Protein Nanopatterns by Combining Nanoimprint Lithography and Molecular Self-Assembly

The electronic structure of the honeycomb lattice iridates ${\mathrm{Na}}_{2}{\mathrm{IrO}}_{3}$ and ${\mathrm{Li}}_{2}{\mathrm{IrO}}_{3}$ has been investigated using resonant inelastic x-ray scattering (RIXS). Crystal-field-split $d\mathrm{\text{\ensuremath{-}}}d$ excitations are resolved in the high-resolution RIXS spectra. In particular, the splitting due to noncubic crystal fields, derived from the splitting of ${j}_{\mathrm{eff}}=3/2$ states, is much smaller than the typical spin-orbit energy scale in iridates, validating the applicability of ${j}_{\mathrm{eff}}$ physics in ${A}_{2}{\mathrm{IrO}}_{3}$. We also find excitonic enhancement of the particle-hole excitation gap around 0.4 eV, indicating that the nearest-neighbor Coulomb interaction could be large. These findings suggest that both ${\mathrm{Na}}_{2}{\mathrm{IrO}}_{3}$ and ${\mathrm{Li}}_{2}{\mathrm{IrO}}_{3}$ can be described as spin-orbit Mott insulators, similar to the square lattice iridate ${\mathrm{Sr}}_{2}{\mathrm{IrO}}_{4}$.

https://link.aps.org/accepted/10.1103/PhysRevLett.110.076402

Crystal-Field Splitting and Correlation Effect on the Electronic Structure of A2IrO3

The electronic structure of Mott insulators continues to be a major unsolved problem in physics despite more than 50 years of research. Well-developed momentum-resolved spectroscopies such as photoemission or neutron scattering cannot probe the full Mott gap. High-resolution resonant inelastic x-ray scattering revealed dispersive charge excitations across the Mott gap in a high–critical temperature parent cuprate (Ca2CuO2Cl2), shedding light on the anisotropy of the Mott gap. These charge excitations across the Mott gap can be described within the framework of the Hubbard model.

https://science.sciencemag.org/content/sci/288/5472/1811.full.pdf

Electronic structure of Mott insulators studied by inelastic X-ray scattering

We report momentum-resolved charge excitations in a one-dimensional (ID) Mott insulator studied using high resolution inelastic x-ray scattering over the entire Brillouin zone for the first time. Excitations at the insulating gap edge are found to be highly dispersive (momentum dependent) compared to excitations observed in two-dimensional Mott insulators. The observed dispersion in ID cuprates (SrCuO 2 and Sr 2 CuO 3 ) is consistent with charge excitations involving holons which is unique to spin-½ quantum chain systems. These results point to the potential utility of momentum-resolved inelastic x-ray scattering in providing valuable information about electronic structure of strongly correlated insulators.

Momentum-resolved charge excitations in a prototype one-dimensional mott insulator.

We briefly report momentum-resolved charge excitations in a cuprate antiferromagnetic insulator in the intermediate regimes of momentum transfers using high energy resonant X-ray scattering near the Cu K-edge. The excitation spectra show dispersive features near the insulating gap edge which can be understood in terms of antiferromagnetic correlation of the underlying lattice. Details of the experiment will be reported elsewhere.

Charge excitations in a quantum antiferromagnet

We briefly report results of an x-ray scattering study of particle-hole excitations in a cuprate insulator. The excitation spectrum shows a broad feature around 5.7 eV and a dispersive feature around 2.5 eV energy-loss. The low energy feature disperses upward about 0.8 eV along 〈110〉 having momentum-transfers (Q) from (2.1π,2.1π) to (1.2π, 1.2π); whereas, it does not disperse much along 〈100〉 in going from (1.42π,0) to (2.85π,0) within the level of energy resolution of 0.56 eV. This dispersive behavior of particle-hole excitations is mostly determined by the underlying strong magnetic correlations.

E.D. Isaacs

Papers

Electronic structure of Mott insulators studied by inelastic X-ray scattering

Momentum-resolved charge excitations in a prototype one-dimensional mott insulator.

Charge excitations in a quantum antiferromagnet

Particle-hole excitations in insulating antiferromagnet Ca2CuO2Cl2

Inelastic X-ray scattering as a novel tool to study electronic excitations in complex insulators