Yunfei Chen

Friction-induced nanofabrication on monocrystalline silicon

Fabrication of nanostructures has become a major concern as the scaling of device dimensions continues. In this paper, a friction-induced nanofabrication method is proposed to fabricate protrusive nanostructures on silicon. Without applying any voltage, the nanofabrication is completed by sliding an AFM diamond tip on a sample surface under a given normal load. Nanostructured patterns, such as linear nanostructures, nanodots or nanowords, can be fabricated on the target surface. The height of these nanostructures increases rapidly at first and then levels off with the increasing normal load or number of scratching cycles. TEM analyses suggest that the friction-induced hillock is composed of silicon oxide, amorphous silicon and deformed silicon structures. Compared to the tribochemical reaction, the amorphization and crystal defects induced by the mechanical interaction may have played a dominating role in the formation of the hillocks. Similar to other proximal probe methods, the proposed method enables fabrication at specified locations and facilitates measuring the dimensions of nanostructures with high precision. It is highlighted that the fabrication can also be realized on electrical insulators or oxide surfaces, such as quartz and glass. Therefore, the friction-induced method points out a new route in fabricating nanostructures on demand.

Direction Dependence of Resistive-Pulse Amplitude in Conically Shaped Mesopores.

Conically shaped pores such as glass pipets as well as asymmetric pores in polymers became an important analytics tool used for the detection of molecules, viruses, and particles. Electrokinetic or pressure driven passage of single particles through a single pore causes a transient change of the transmembrane current, called a resistive-pulse, whose amplitude is the measure of the particle volume. The shape of the pulse reflects the pore topography, and in a conical pore, resistive pulses have a shape of a tick point. Passage of particles in both directions was reported to produce pulses of the same amplitude and shapes that are mirror images of each other. In this manuscript we identify conditions at which the amplitude of resistive-pulses in a conical mesopore is direction dependent. Neutral particles entering the pore from the larger entrance of a conical pore, called the base, block the current to a larger extent than the particles traveling in the opposite direction. Negatively charged particles on the other hand size larger when being transported in the direction from tip to base. The findings are explained via voltage-regulated ionic concentrations in the pore such that for one voltage polarity a weak depletion zone is formed, which increases the current blockage caused by a particle. For the opposite polarity, an enhancement of ionic concentrations was predicted. The findings reported here are of crucial importance for the resistive-pulse technique, which relates the current blockage with the size of the passing object.

Optimal design of graphene nanopores for seawater desalination

Extensive molecular dynamics simulations are employed to optimize nanopore size and surface charge density in order to obtain high ionic selectivity and high water throughput for seawater desalination systems. It is demonstrated that with the help of surface charge exclusion, nanopores with diameter as large as 3.5 nm still have high ionic selectivity. The mechanism of the salt rejection in a surface-charged nanopore is mainly attributed to the ion concentration difference between the cations and anions induced by the surface charges. Increasing surface charge density is beneficial to enhance ionic selectivity. However, there exists a critical value for the surface charge density. Once the surface charge density exceeds the critical value, charge inversion occurs inside a nanopore. Further increasing the surface charge density will deteriorate the ionic selectivity because the highly charged nanopore surface will allow more coions to enter the nanopore in order to keep the whole system in charge neutrality. Besides the surface charge density, the nanopore length also affects the ionic selectivity. Based on our systematic simulations, nanopores with surface charge density between -0.09 C/m2 and -0.12 C/m2, diameters smaller than 3.5 nm, and membrane thickness ranging between 8 and 10 graphene layers show an excellent performance for the ionic selectivity.

Salt Gradient Improving Signal-to-Noise Ratio in Solid-State Nanopore

As the single molecule detection tool, solid-state nanopores are being applied in more and more fields, such as medicine controlled delivery, ion conductance microscopes, nanosensors, and DNA sequencing. The critical information obtained from nanopores is the signal collected, which is the ionic block current caused by the molecules passing through the pores. However, the information collected is, in part, impeded by the relatively low signal-to-noise ratio of the current solid-state nanopore measurements. Here, we report that using a salt gradient across the nanopore could improve the signal-to-noise ratio when molecules translocate through Si3N4 nanopore. Furthermore, we demonstrate that the improved signal-to-noise ratio is connected with not only the value of surface charge but also that of a salt gradient between cis and trans sides of the nanopore.

Identification of Single Nucleotides by a Tiny Charged Solid-State Nanopore

Discrimination of single nucleotides by a nanopore remains a challenge because of the minor difference among the four types of single nucleotides. Here, the blockade currents induced by the translocation of single nucleotides through a 1.8 nm diameter silicon nitride nanopore have been measured. It is found that the single nucleotides are driven through the nanopore by an electroosmotic flow instead of electrophoretic force when a bias voltage is applied. The blockade currents for the four types of single nucleotides are unique and differentiable, following the order of the nucleotide volume. Also, the dwell time for each single nucleotide can last for several hundred microseconds with the advantage of the electroosmotic flow, which is helpful for single nucleotide identification. The dwell-time distributions are found to obey the first-passage time distribution from the 1D Fokker-Planck equation, from which the velocity and diffusion constant of each nucleotide can be deduced. Interestingly, the larger nucleotide is found to translocate faster than the smaller one inside the nanopore because the larger nucleotide has a larger surface area, which may produce larger drag force induced by the electroosmotic flow, which is validated by molecular dynamics simulations.

Selective ion-permeation through strained and charged graphene membranes

By means of molecular dynamics simulations and density functional theory calculations, we demonstrate that stretched and charged graphene can act as ion sieve membranes. It is observed that loading 30% strain on graphene can induce pores in the dense electron cloud to allow ions to pass through the aromatic rings. Meanwhile, a charged surface is helpful to peel the hydration layers from the ions and decrease the energy barrier for ion translocation through nanopores. Our results suggest that with a membrane charge density of 6.80 e nm-2, Li+ can be highly purified from the mixed solution including Li+, K+, Na+ and Cl- ions. Further increasing the charge density to 15.78 e nm-2 can obtain excellent Na+/K+ selectivity. The potential of mean force profiles of ion permeation reveal that the potential for each ion is quite different. By fine tuning membrane charge density, pristine monolayer graphene can act as ion sieves with both high permeability and high selectivity.