In the current research environment, the potential contribution (and guidance) of aspiring undergraduates is often neglected. Here, Ye Tao fromthe Rowland Institute discusses the opportunities and challenges to foster this next generation of scientists and proposes three guiding principles of successful undergraduate mentorship.
High saturation magnetization and hysteresis-less magnetic responses are desirable for nanoparticles in scientific and technological applications. Rare-earth oxides are potentially promising materials because of their paramagnetism and high magnetic susceptibility in the bulk, but the magnetic properties of their nanoparticles remain incompletely characterized. Here, we present full M–H loops for commercial RE2O3 nanoparticles (RE = Er, Gd, Dy, Ho) with radii from 10–25 nm at room temperature and 4 K. The magnetic responses are consistent with two distinct populations of atoms, one displaying the ideal Re3þ magnetic moment and the other displaying a sub-ideal magnetic moment. If all sub-ideal ions are taken to be on the surface, the data are consistent with 2-10 nm surface layers of reduced magnetization. The magnetization of the rare-earth oxide nanoparticles at low temperatures (1.3–1.9 T) exceeds that of the best iron-based nanoparticles, making rare-earth oxides candidates for use in next-generation cryogenic magnetic devices that demand a combination of hysteresis-less response and high magnetization.
High saturation magnetization, hysteresis-less long linear response range, and resistance to device fabrication conditions are figures of merit for magnetic materials. Despite advances in materials research, many high-saturating micro- and nanomagnetic materials are hysteresis-prone, have short linear ranges, and are sensitive to oxidation, resulting in device inefficiencies in high-frequency electronics and unpredictable responses in magnetic sensing applications. Holmium oxide is a promising material because of its high magnetic susceptibility, but synthetic options are limited, with low-temperature magnetism incompletely characterized. Here, we present physical vapor deposition synthesis and material characterization of polycrystalline holmium oxide thin films. The product has saturation magnetization exceeding 2 Tesla, linear range (m0H) also exceeding 2 Tesla, zero magnetic remanence and coercivity, and resistance to harsh processing conditions including oxygen plasma and concentrated hydrofluoric acid etching, making these thin films ideal for fabricating next-generation nanoscale magnetic devices in advanced scientific and engineering applications.
We present a “nanoladder” geometry that minimizes the mechanical dissipation of ultrasensitive cantilevers. A nanoladder cantilever consists of a lithographically patterned scaffold of rails and rungs with feature size ∼100 nm. Compared to a rectangular beam of the same dimensions, the mass and spring constant of a nanoladder are each reduced by roughly 2 orders of magnitude. We demonstrate a low force noise of 158 (−42, +62) zN and 190 (−33, +42) zN in a 1 Hz bandwidth for devices made from silicon and diamond, respectively, measured at temperatures between 100−150 mK. As opposed to bottom-up mechanical resonators like nanowires or nanotubes, nanoladder cantilevers can be batchfabricated using standard lithography, which is a critical factor for applications in scanning force microscopy.
The detection of small forces using singly clamped cantilevers is a fundamental feature in ultrasensitive versions of scanning probe force microscopy. In these technologies, silicon-based nanomechanical devices continue to be the most widespread high-performance nanomechanical sensors for their availability, ease of fabrication, inherently low mechanical dissipation, and good control of surface-induced mechanical dissipation. Here, we develop a robust method to batch fabricate extreme-aspect-ratio (103), singly clamped scanning nanowire mechanical resonators from plain bulk silicon wafers using standard photolithography. We discuss the superior performance and additional versatility of the approach beyond what can be achieved using the established silicon on insulator technology.
The production of nanowire materials, uniformly oriented along any arbitrarily chosen crystal orientation, is an important, yet unsolved, problem in material science. Here, we present a generalizable solution to this problem. The solution is based on the technique of glancing angle deposition combined with a rapid switching of the deposition direction between crystal symmetry positions. Using iron–cobalt as an example, we showcase the simplicity and capabilities of the process in one-step fabrications of 〈100〉, 〈110〉, 〈111〉, 〈210〉, 〈310〉, 〈320〉, and 〈321〉-oriented nanowires, three-dimensional nanowire spirals, core–shell heterostructures, and axial hybrids. Our results provide a new capability for tailoring the properties of nanowires, and should be generalizable to any material that can be grown as a single crystal biaxial film.
Ferromagnetic nanowires are finding use as untethered sensors and actuators for probing micro- and nanoscale biophysical phenomena, such as for localized sensing and application of forces and torques on biological samples, for tissue heating through magnetic hyperthermia, and for microrheology. Quantifying the magnetic properties of individual isolated nanowires is crucial for such applications. Dynamic cantilever magnetometry is used to measure the magnetic properties of individual sub-500 nm diameter polycrystalline nanowires of Ni and Ni80Co20 fabricated by template-assisted electrochemical deposition. The values are compared with bulk, ensemble measurements when the nanowires are still embedded within their growth matrix. It is found that single-particle and ensemble measurements of nanowires yield significantly different results that reflect inter-nanowire interactions and chemical modifications of the sample during the release process from the growth matrix. The results highlight the importance of performing single-particle characterization for objects that will be used as individual magnetic nanoactuators or nanosensors in biomedical applications.
Reactions in the gas phase are of primary technological importance for applications in nano- and microfabrication technology and in the semiconductor industry. We present exclusively gas-phase protocols to chemically passivate oxide-free Si(111) and Si(100) surfaces with short-chain alkynes. The resulting surfaces showed equal or better oxidation resistance than most existing liquid-phasederived surfaces and rivaled the outstanding stability of a fullcoverage Si(111)−propenyl surface. The most stable surface (Si(111)−ethenyl) grew one-fifth of a monolayer of oxide (0.04 nm) after 1 month of air exposure. We monitored the regrowth of oxides on passivated Si(111) and Si(100) surfaces by X-ray photoelectron spectroscopy (XPS) and observed a significant crystal-orientation dependence of initial rates when total oxide thickness was below approximately one monolayer (0.2 nm). This difference was correlated with the desorption kinetics of residual surface Si−F bonds formed during HF treatment. We discuss applications of the technology and suggest future directions for process optimization.
We report electronic and phononic transport measurements of monocrystalline batch-fabricated silicon nanowire (SiNW) arrays functionalized with different surface chemistries. We find that hydrogen-terminated SiNWs prepared by vapor HF etching of native-oxide-covered devices show increased electrical conductivity but decreased thermal conductivity. We used the kinetic Monte Carlo method to solve the Boltzmann transport equation and also numerically examine the effect of phonon boundary scattering. Surface transfer doping of the SiNWs by cobaltocene or decamethylcobaltocene drastically improves the electrical conductivity by 2 to 4 orders of magnitude without affecting the thermal conductivity. The results showcase surface chemical control of nanomaterials as a potent pathway that can complement device miniaturization efforts in the quest for more efficient thermoelectric materials and devices.
Sensitive detection of weak magnetic moments is an essential capability in many areas of nanoscale science and technology, including nanomagnetism, quantum readout of spins and nanoscale magnetic resonance imaging. Here we show that the write head of a commercial hard drive may enable significant advances in nanoscale spin detection. By approaching a sharp diamond tip to within 5 nm from a write pole and measuring the induced diamagnetic moment with a nanomechanical force transducer, we demonstrate a spin sensitivity of 0.032 μB Hz−1/2, equivalent to 21 proton magnetic moments. The high sensitivity is enabled in part by the pole’s strong magnetic gradient of up to 28 × 106 T m−1 and in part by the absence of non-contact friction due to the extremely flat writer surface. In addition, we demonstrate quantitative imaging of the pole field with ∼10 nm spatial resolution. We foresee diverse applications for write heads in experimental condensed matter physics, especially in spintronics, ultrafast spin manipulation and mesoscopic physics.
We report a method for accelerated nanoscale nuclear magnetic resonance imaging by detecting several signals in parallel. Our technique relies on phase multiplexing, where the signals from different nuclear spin ensembles are encoded in the phase of an ultrasensitive magnetic detector. We demonstrate this technique by simultaneously acquiring statistically polarized spin signals from two different nuclear species (1H, 19F) and from up to six spatial locations in a nanowire test sample using a magnetic resonance force microscope. We obtain one-dimensional imaging resolution better than 5 nm, and subnanometer positional accuracy.
We report on mechanical dissipation measurements carried out on thin (∼100 nm), single-crystal silicon cantilevers with varying chemical surface termination. We find that the 1–2 nm-thick native oxide layer of silicon contributes about 85% to the friction of the mechanical resonance. We show that the mechanical friction is proportional to the thickness of the oxide layer and that it crucially depends on oxide formation conditions. We further demonstrate that chemical surface protection by nitridation, liquid-phase hydrosilylation, or gas-phase hydrosilylation can inhibit rapid oxide formation in air and results in a permanent improvement of the mechanical quality factor between three- and five-fold. This improvement extends to cryogenic temperatures. Presented recipes can be directly integrated with standard cleanroom processes and may be especially beneficial for ultrasensitive nanomechanical force- and mass sensors, including silicon cantilevers, membranes, or nanowires.
We report the fabrication, integration, and assessment of sharp diamond tips for ultrasensitive force microscopy experiments. Two types of tips, corresponding to the upper and lower halves of a diamond nanowire, were fabricated by top-down plasma etching from a single-crystalline substrate. The lower, surface-attached halves can be directly integrated into lithographically defined nanostructures, like cantilevers. The upper, detachable halves result in diamond nanowires with a tunable diameter (50−500 nm) and lengths of a few microns. Tip radii were around 10 nm and tip apex angles around 15°. We demonstrate the integration of diamond nanowires for use as scanning tips onto ultrasensitive pendulum-style silicon cantilevers. We find the noncontact friction and frequency jitter to be exceptionally low, with no degradation in the intrinsic mechanical quality factor (Q ≈ 130 000) down to tip-to-surface distances of about 10 nm. Our results are an encouraging step toward further improvement of the sensitivity and resolution of force-detected magnetic resonance imaging.
Diamond has gained a reputation as a uniquely versatile material, yet one that is intricate to grow and process. Resonating nanostructures made of single-crystal diamond are expected to possess excellent mechanical properties, including high-quality factors and low dissipation. Here we demonstrate batch fabrication and mechanical measurements of single-crystal diamond cantilevers with thickness down to 85 nm, thickness uniformity better than 20 nm and lateral dimensions up to 240 mm. Quality factors exceeding one million are found at room temperature, surpassing those of state-of-the-art single-crystal silicon cantilevers of similar dimensions by roughly an order of magnitude. The corresponding thermal force noise for the best cantilevers is B5 1019NHz1/2 at millikelvin temperatures. Single-crystal diamond could thus directly improve existing force and mass sensors by a simple substitution of resonator material. Presented methods are easily adapted for fabrication of nanoelectromechanical systems, optomechanical resonators or nanophotonic devices that may lead to new applications in classical and quantum science.
A robust and facile approach for making single-crystal-diamond MEMS and NEMS devices is presented. The approach relies entirely on commercial diamond material and standard cleanroom processes. As an example, batch fabrication of cantilever beams of thickness down to 45 nm and aspect ratios exceeding 2000:1 is demonstrated.
We report successful introduction of negatively charged nitrogen-vacancy (NV–) centers in a 5 nm thin, isotopically enriched ([12C] = 99.99%) diamond layer by CVD. The present method allows for the formation of NV– in such a thin layer even when the surface is terminated by hydrogen atoms. NV– centers are found to have spin coherence times of between T2 ∼ 10–100 μs at room temperature. Changing the surface termination to oxygen or fluorine leads to a slight increase in the NV– density, but not to any significant change in T2. The minimum detectable magnetic field estimated by this T2 is 3 nT after 100 s of averaging, which would be sufficient for the detection of nuclear magnetic fields exerted by a single proton. We demonstrate the suitability for nanoscale NMR by measuring the fluctuating field from ∼104 proton nuclei placed on top of the 5 nm diamond film.
Spontaneous asymmetric generation of supramolecular chiral fibers was observed in the folding induced self-assembly of a lock-washer shaped foldamer. A secondary nucleation growth mechanism is proposed to explain the observed chiral amplification or deracemization of these supramolecular fibers.
We investigate spin and optical properties of individual nitrogen vacancy centers located within 1–10 nm from the diamond surface. We observe stable defects with a characteristic optically detected magnetic-resonance spectrum down to the lowest depth.We also find a small but systematic spectral broadening for defects shallower than about 2 nm. This broadening is consistent with the presence of a surface paramagnetic impurity layer [Tisler et al., ACS Nano 3, 1959 (2009)] largely decoupled by motional averaging. The observation of stable and well-behaved defects very close to the surface is critical for single-spin sensors and devices requiring nanometer proximity to the target.