David Miller

Many techniques to fabricate complex nanostructures and quantum emitting defects in low dimensional materials for quantum information technologies rely on the patterning capabilities of focused ion beam (FIB) systems. In particular, the ability to pattern arrays of bright and stable room temperature single-photon emitters (SPEs) in 2D wide-bandgap insulator hexagonal boron nitride (hBN) via high-energy heavy-ion FIB allows for direct placement of SPEs without structured substrates or polymer-reliant lithography steps. However, the process parameters needed to create hBN SPEs with this technique are dependent on the growth method of the material chosen. Moreover, morphological damage induced by high-energy heavy-ion exposure may further influence the successful creation of SPEs. In this work, we perform atomic force microscopy to characterize the surface morphology of hBN regions patterned by Ga⁺ FIB to create SPEs at a range of ion doses and find that material swelling, and not milling as expected, is most strongly and positively correlated with the onset of non-zero SPE yields. Furthermore, we simulate vacancy concentration profiles at each of the tested doses and propose a qualitative model to elucidate how Ga⁺ FIB patterning creates isolated SPEs that is consistent with observed optical and morphological characteristics and is dependent on the consideration of void nucleation and growth from vacancy clusters. Our results provide novel insight into the formation of hBN SPEs created by high-energy heavy-ion milling that can be leveraged for monolithic hBN photonic devices and could be applied to a wide range of low-dimensional solid-state SPE hosts.

Arrays of nanoelectromechanical resonators (NEMS) have shown promise for a suite of applications, from nanomechanical information processing technologies to mass spectrometry. A fundamental challenge towards broader adoption of NEMS arrays is a lack of viable frequency tuning methods, which must simultaneously allow for persistent and reversible control of single resonators while also being scalable to large arrays of devices. In this work, we demonstrate an electro-optic tuning method for graphene-based NEMS where locally photo-ionized charge tensions a suspended membrane and tunes its resonance frequency. The tuned frequency state persists unchanged for several days in the absence of any external power, and the state can be repeatedly written and erased with a high degree of precision. We show the scalability of this technique by aligning the frequencies of several NEMS devices on the same chip, and we discuss implications of this tuning method for both single devices and programmable NEMS networks.

Raman spectroscopy can provide a detailed analysis of mechanical stress in crystalline materials through the measurement of shifts in vibrational frequency. However, the ambient temperature drift during measurements can lead to inaccurate mechanical stress values. Herein, the 2D Raman mapping analysis of gallium nitride (GaN) and aluminum indium gallium phosphide (AlInGaP) epitaxial films within packaged light‐emitting diode (LED) devices reveals micrometer‐scale localized stress distributions. A temperature‐corrected measurement method is developed to evaluate the mechanical stress of InGaN LED dies at various LED operating conditions. The impact of die design on mechanical stress is studied for both GaN on Al₂O₃ (chip scale package, CSP) and GaN thin‐film flip chip (TFFC) designs. Raman mapping of AlInGaP LEDs is also demonstrated and shows comparable resolution with the GaN LED results.

Emerging applications in nanoelectromechanical systems (NEMS) made from two-dimensional (2D) materials demand simultaneous imaging and selective actuation of the mechanical modes. Focused optical probes to measure and actuate motion offer a possible solution, but their lateral spatial resolution must be better than the size of the resonator. While optical interferometry is known to have excellent spatial resolution, the spatial resolution of the focused, laser-based optical driving is not currently known. Here, we combine separately scanned interferometry and optical drive probes to map the motion and forces on a suspended graphene nanomechanical resonator. By analyzing these maps with a force density model, we determine that the optical drive force has a spatial resolution on the order of the size of the focused laser spot. Using the optical force probe, we demonstrate the selective actuation and suppression of a pair of orthogonal antisymmetric mechanical modes of the graphene resonator. Our results offer a powerful approach to image and actuate any arbitrary high-order mode of a 2D NEMS.

Bolometers are a powerful means of detecting light. Emerging applications demand that bolometers work at room temperature, while maintaining high speed and sensitivity, properties which are inherently limited by the heat capacity of the detector. To this end, graphene has generated interest, because it has the lowest mass per unit area of any material, while also possessing extreme thermal stability and an unmatched spectral absorbance. Yet, due to its weakly temperature-dependent electrical resistivity, graphene has failed to challenge the state-of-the-art at room temperature. Here, in a departure from conventional bolometry, we use a graphene nanoelectromechanical system to detect light via resonant sensing. In our approach, absorbed light heats and thermally tensions a suspended graphene resonator, thereby shifting its resonant frequency. Using the resonant frequency as a readout for photodetection, we achieve a room-temperature noise-equivalent power (2 pW Hz^−1/2) and bandwidth (from 10 kHz up to 1.3 MHz), challenging the state-of-the-art.

Quantum emitters (QEs) in 2D hexagonal boron nitride (hBN) are extremely bright and are stable at high temperature and under harsh chemical conditions. Because of they reside within an atomically thin 2D material, these QEs have a unique potential to couple strongly to hybrid optoelectromechanical and quantum devices. However, this potential for coupling has been underexplored because of challenges in nanofabrication and patterning of hBN QEs. Motivated by recent studies showing that QEs in hBN tend to form at edges, we use a focused ion beam (FIB) to mill an array of patterned holes into hBN. Using optical confocal microscopy, we find arrays of bright, localized photoluminescence that match the geometry of the patterned holes. Furthermore, second-order photon correlation measurements on these bright spots reveal that they contain single and multiple QEs. By optimizing the FIB parameters, we create patterned single QEs with a yield of 31%, a value close to Poissonian limit. Using atomic force microscopy to study the morphology near emission sites, we find that single QE yield is highest with smoothly milled holes on unwrinkled hBN. This technique dramatically broadens the utility and convenience of hBN QEs, and achieves a vital step toward the facile integration of the QEs into large-scale photonic, plasmonic, nanomechanical, or optoelectronic devices.

Extended carbon nanostructures, such as carbon nanotubes (CNTs), exhibit remarkable properties but are difficult to synthesize uniformly. Herein, we present a new class of carbon nanomaterials constructed via the bottom-up self-assembly of cylindrical, atomically-precise small molecules. Guided by supramolecular design principles and circle packing theory, we have designed and synthesized a fluorinated nanohoop that, in the solid-state, self-assembles into nanotube-like arrays with channel diameters of precisely 1.63 nm. A mild solution-casting technique is then used to construct vertical “forests” of these arrays on a highly-ordered pyrolytic graphite (HOPG) surface through epitaxial growth. Furthermore, we show that a basic property of nanohoops, fluorescence, is readily transferred to the bulk phase, implying that the properties of these materials can be directly altered via precise functionalization of their nanohoop building blocks. The strategy presented is expected to have broader applications in the development of new graphitic nanomaterials with π-rich cavities reminiscent of CNTs.

Quantum emitters in two-dimensional hexagonal boron nitride (hBN) are attractive for a variety of quantum and photonic technologies because they combine ultra-bright, room-temperature single-photon emission with an atomically thin crystal. However, the emitter’s prominence is hindered by large, strain-induced wavelength shifts. We report the discovery of a visible-wavelength, single-photon emitter (SPE) in a zero-dimensional boron nitride allotrope (the boron nitride nanococoon, BNNC) that retains the excellent optical characteristics of few-layer hBN while possessing an emission line variation that is lower by a factor of 5 than the hBN emitter. We determined the emission source to be the nanometer-size BNNC through the cross-correlation of optical confocal microscopy with high-resolution scanning and transmission electron microscopy. Altogether, this discovery enlivens color centers in BN materials and, because of the BN nanococoon’s size, opens new and exciting opportunities in nanophotonics, quantum information, biological imaging, and nanoscale sensing.