M. G. Lagally's research while affiliated with University of Wisconsin–Madison and other places
What is this page?
This page lists the scientific contributions of an author, who either does not have a ResearchGate profile, or has not yet added these contributions to their profile.
It was automatically created by ResearchGate to create a record of this author's body of work. We create such pages to advance our goal of creating and maintaining the most comprehensive scientific repository possible. In doing so, we process publicly available (personal) data relating to the author as a member of the scientific community.
If you're a ResearchGate member, you can follow this page to keep up with this author's work.
If you are this author, and you don't want us to display this page anymore, please let us know.
This work demonstrates that the electrodeposition of highly stressed films on compliant ribbons is a robust process to obtain helical structures with excellent mechanical stability and potentially high thermal and electrical conductance. Electrodeposition on end‐tethered ribbons alters their axial and bending stiffness while imparting mechanical stress to drive the formation of a helix with a microscale diameter and pitch in a controlled and scalable manner. The process generates helices with diameters and pitches between 80 and 200 µm and lengths as large as several millimeters. The approach is amenable to parallel processing a large number of 3D structures on any substrate, including large‐area semiconductor wafers. This phenomenon is explained in terms of the change of stress gradients as material is added. Applications of the fabricatd helices include antennas, metamaterials, and slow‐wave structures in frequency ranges not previously attainable.
We report gold electroplating on ultra-compliant substrates comprising helical slow wave structures (SWSs) for traveling wave tube amplifiers (TWTAs) [1]. The novel ultra-compliant substrates are composed of edge-tethered tri-layer metal ribbons with a helical geometry of microscale diameter. After electroplating with gold, we obtain overall thicknesses of a few um. We discuss different controllable electroplating conditions that influence thickness, uniformity, roughness, and related properties of deposited gold films on helical ribbons. In addition to increasing conductance of the electroplated helical ribbons, electroplating stabilizes the helix at its equilibrium diameter, with the pitch predicted by Prakash et al .[1].
Our method of fabrication of ultra-compliant helical ribbons starts with defining strips of width 5 µm to 10 µm by optical lithography, metal evaporation, and lift-off, deposited on Si substrates coated with a sacrificial layer of Ge or GeOx,. We use Cr/Au/Cr tri-layers to create an inherent stress gradient that causes the strip to self-assemble into a helix, after etching with XeF 2 for selective removal of a sacrificial layer. Diameter and pitch of the released helices are controlled by varying the thickness, the elastic modulus, the residual stress, and the in-plane geometry of the deposited tri-layer metal strips. The gold electroplating process uses a sulphate-based gold solution in a two-electrode electrochemical setup with the helix as the cathode and a platinized mesh as the anode [2]. We deposit gold to a thickness of a few um, using a pulsed current source with variable parameters. Direct current can also be used with smaller deposition times. Our results demonstrate the application of electroplating to unconventional ultra-compliant helix of nanoscale dimensions.
Reference
[1] Divya J. Prakash,., Matthew M. Dwyer, Marcos Martinez Argudo, Mengistie L. Debasu, Hassan Dibaji, Max G. Lagally, Daniel W. van der Weide, and Francesca Cavallo. 2020. “Self-Winding Helices as Slow-Wave Structures for Sub-Millimeter Traveling-Wave Tubes.” ACS Nano, 2021, 15, 1229-1239. doi:10.1021/acsnano.0c08296.
[2] Max G. Lagally, Anjali Chaudhary, Daniel van der Weide, Divya J. Prakash, and Francesca Cavallo, Improved Self-assembly of Helices via Electrodeposition on Freestanding Nanoribbons for TWT Application, provisional US patent application (P220249US01)
Work supported by U.S. AFOSR-Award No FA9550-22-1-0086.
Submicrometer-thick layers of hexagonal boron nitride (hBN) exhibit high in-plane thermal conductivity and useful optical properties, and serve as dielectric encapsulation layers with low electrostatic inhomogeneity for graphene devices. Despite the promising applications of hBN as a heat spreader, the thickness dependence of its cross-plane thermal conductivity is not known, and the cross-plane phonon mean free paths (MFPs) have not been measured. We measure the cross-plane thermal conductivity of hBN flakes exfoliated from bulk crystals. We find that submicrometer thick flakes exhibit thermal conductivities up to 8.1 ± 0.5 W m-1 K-1 at 295 K, which exceeds previously reported bulk values by more than 60%. Surprisingly, the average phonon mean free path is found to be several hundred nanometers at room temperature, a factor of 5 larger than previous predictions. When planar twist interfaces are introduced into the crystal by mechanically stacking multiple thin flakes, the cross-plane thermal conductivity of the stack is found to be a factor of 7 below that of individual flakes with similar total thickness, thus providing strong evidence that phonon scattering at twist boundaries limits the maximum phonon MFPs. These results have important implications for hBN integration in nanoelectronics and improve our understanding of thermal transport in two-dimensional materials.
Large-scale arrays of quantum-dot spin qubits in Si/SiGe quantum wells require large or tunable energy splittings of the valley states associated with degenerate conduction band minima. Existing proposals to deterministically enhance the valley splitting rely on sharp interfaces or modifications in the quantum well barriers that can be difficult to grow. Here, we propose and demonstrate a new heterostructure, the “Wiggle Well”, whose key feature is Ge concentration oscillations inside the quantum well. Experimentally, we show that placing Ge in the quantum well does not significantly impact our ability to form and manipulate single-electron quantum dots. We further observe large and widely tunable valley splittings, from 54 to 239 μeV. Tight-binding calculations, and the tunability of the valley splitting, indicate that these results can mainly be attributed to random concentration fluctuations that are amplified by the presence of Ge alloy in the heterostructure, as opposed to a deterministic enhancement due to the concentration oscillations. Quantitative predictions for several other heterostructures point to the Wiggle Well as a robust method for reliably enhancing the valley splitting in future qubit devices. Quantum-dot spin qubits in Si/SiGe quantum wells require a large and uniform valley splitting for robust operation and scalability. Here the authors introduce and characterize a new heterostructure with periodic oscillations of Ge atoms in the quantum well, which could enhance the valley splitting.
We investigate the interaction between an electron beam and a THz guided electromagnetic wave in a helical slow-wave structure formed by self-assembly of a conductive ribbon. We have previously shown the controlled fabrication of this slow-wave structure and its potential to form the basis for widely deployable millimeter-through-THz traveling-wave tube amplifiers. The process allows the fabrication of helical slow-wave structures with single and double chirality. Here, we use three-dimensional simulations to perform a comparative analysis of beam–wave interaction in self-assembled gold helices with single and double chirality. First, the structures are modeled without the electron beam (cold helices) to calculate the distribution of the electric field generated by the high-frequency wave. We perform simulations of cold helices by using Computer Simulation Technology Microwave Studio. Second, we evaluate the interaction between an electron beam and the THz travelingwave by using a particle in cell simulator in Computer Simulation Technology Particle Studio. Simulation studies show that a switch in chirality in the middle of self-assembled helices generates a reflected wave that boosts beam–wave interaction. We demonstrate that this efficient energy exchange will potentially provide high gain in THz traveling-wave tube amplifiers based on self-assembled helices.
The current autotuning approaches for quantum dot (QD) devices, while showing some success, lack an assessment of data reliability. This leads to unexpected failures when noisy or otherwise low-quality data is processed by an autonomous system. In this work, we propose a framework for robust autotuning of QD devices that combines a machine learning (ML) state classifier with a data quality control module. The data quality control module acts as a “gatekeeper” system, ensuring that only reliable data are processed by the state classifier. Lower data quality results in either device recalibration or termination. To train both ML systems, we enhance the QD simulation by incorporating synthetic noise typical of QD experiments. We confirm that the inclusion of synthetic noise in the training of the state classifier significantly improves the performance, resulting in an accuracy of 95.0(9)% when tested on experimental data. We then validate the functionality of the data quality control module by showing that the state classifier performance deteriorates with decreasing data quality, as expected. Our results establish a robust and flexible ML framework for autonomous tuning of noisy QD devices.
... The weak interlayer van der Waals forces limit high-frequency vibration modes, which significantly suppresses the phonon transmissivity along the cross-plane directions. [4][5][6] Moreover, heat transfer through interfaces comprising 2DM has also attracted great attention, since 2DM have been widely investigated for potential applications in electronic, photonic or spintronic devices. For example, the few-layer h-BN has been used as the dielectric layer in resistive random-access memory (RRAM) 7 , and as the tunnel barrier layer in magnetic tunnel junction (MTJ) 8 . ...
... Quantum confinement in direct bandgap semiconductors has stood at the cradle of many photonic devices such as single photon quantum dot (QD) emitters [24][25][26][27] , quantum well (QW) lasers 28,29 and colloidal QD LED display technology [30][31][32] . These direct bandgap low dimensional structures have been responsible for major advances in science and constitute a toolbox for many optoelectronic and quantum photonic devices 33,34 , allowing for tunable and narrow band emission, and the concentration of charge carriers. ...
... In this context, the recent findings on the direct growth of single-layer graphene on a Ge surface and their ongoing progress provide an appropriate roadmap toward a potentially integrable interface [19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34]. However, one must pay attention to which Ge surface is used, since the graphene's growth may or may not be accompanied by faceting of the Ge substrate. ...
... Before using QDs as qubits to perform operations, each QD has to be tuned into a specific charge state by applying appropriate gate voltages. The complete calibration procedure can be divided into five distinct steps: (i) bootstrapping: cooling the device and bringing its regime into the appropriate parameter range ; (ii) coarse tuning [25,26,[29][30][31][32][33][34]: tuning the QDs into a specific topology (e.g., single QD, double QD) ; (iii) establishing controllability [26,29,35,36]: setup virtual gates that compensate capacitive cross-talk ; (iv) charge state tuning [22,23,26,37]: tuning the QDs into specific charge configuration (number of electrons in our case) ; (v) fine-tuning [29,38]: adjusting the inter-dot tunnel coupling. For a more detailed description of these steps, refer to Zwolak and Taylor [39]. ...
... Many different methods to determine the E VS of a Si/SiGe QD were reported, such as thermal excitation 8 , pulsed-gate spectroscopy in a single 21,24 or double 23 QD, spin funnel measurement in two exchange-coupled QDs 32,33 and the identification of the spin-valley relaxation hot-spot 20,21 . Other methods measure the singlet-triplet energy splitting E ST , being a lower bound of the E VS , by Pauli-spin blockade 19 or magnetospectroscopy [15][16][17]22,26,31 . High-energy resolution has been achieved by dispersive coupling, to a resonator 18,34 , and some attempts towards laterally mapping E VS 21,24,25 have been published, but these are involved, time-consuming, and cover a small area. ...
... In contrast, synchrotronbased X-ray micro-diffraction techniques allow for a non-destructive investigation of the deformation within the strained regions of such devices 22,23 that can reside at or below the top surface of the sample. Nanodiffraction with sub-100-nm resolution has been reported in studies of various crystalline materials [24][25][26][27] . To date, micro/nanodiffraction implementations possess a spatial resolution often found to be insufficient in studying the structural deformation within modern nanoelectronics. ...
... The radiation tolerance of multiwalled carbon nanotubes (CNTs) is demonstrated by the ability of carbon atoms to be displaced after gamma-ray exposure only in the vicinity of the graphene plane, as the radiation-induced structural rearrangement is restricted to chemical cross-links between the carbon atoms from the nanotube and the nearest carbon atoms from the environment [5,6]. Currently, there are two noticeable trends in the exploitation of the radiation hardness of graphene [7] and other 2D and plane-based materials [8]. On the one hand, as a promising tool for single atom manipulation, the radiation 2D damage provides a basis for nanoscale defect engineering. ...
... Recently to utilize their unique physical properties, helices can be exquisitely fabricated by various techniques, such as 3D printing, [8] glancing angle deposition, [9] ion/electron beaminduced deposition, [10] and DNA-based self-assembly. [11] These processes facilitate the production of helices scaled from millimeters to nanoscale, capable of operating with different functionalities in micro robotics, [12] electronic devices, [13] artificial muscles, [14] and optical sensors. [10b,11,15] Researchers are particularly interested in using chirality for sensing because its unique properties offer sensing capabilities that non-chiral systems do not, making it a promising research field. ...
... After this working frequency, the phase angle is varying abruptly due to the extrinsic characteristic of the material, and it is not advisable to execute the experiment above the working frequency. Also, multiple heaters can be employed to remove the systematic errors due to drift in ambient condition by performing simultaneous measurement from all the heater [102]. ...
... Analyzing the two-qubit system, we consider two distinct possibilities of dynamical fluctuations: Splitting energies J 1 , J 2 could fluctuate independently, i.e., J i (t) =J i + δ J i (t), or their fluctuations may have a common source J i (t) =J i + s i δ J (t), where s i ∈ [0, 1] is a coupling of ith qubit to the noise. Note that correlations of lowfrequency charge noises affecting two quantum dots separated by ∼ 100 nm distance have been observed in experiments [80,88]. Correspondingly, the two-qubit coupling in the former case reads ...