We investigate electrical transport and optoelectronic properties of field effect transistors (FETs) made from few-layer black phosphorus (BP) crystals down to a few nanometers. In particular, we explore the anisotropic nature and photocurrent generation mechanisms in BP FETs through spatial-, polarization-, gate-, and bias-dependent photocurrent measurements. Our results reveal that the photocurrent signals at BP-electrode junctions are mainly attributed to the photovoltaic effect in the off-state and photothermoelectric effect in the on-state, and their anisotropic feature primarily results from the directional-dependent absorption of BP crystals.

We investigate the photocurrent generation mechanisms at a vertical p-n heterojunction between black phosphorus (BP) and molybdenum disulfide (MoS2) flakes through polarization-, wavelength-, and gate-dependent scanning photocurrent measurements. When incident photon energy is above the direct band gap of MoS2, the photocurrent response demonstrates a competitive effect between MoS2 and BP in the junction region. In contrast, if the incident photon energy is below the band gap of MoS2 but above the band gap of BP, the photocurrent response at the p-n junction exhibits the same polarization dependence as that at the BP-metal junction, which is nearly parallel to the MoS2 channel. This result indicates that the photocurrent signals at the MoS2-BP junction primarily result from the direct band gap transition in BP. These fundamental studies shed light on the knowledge of photocurrent generation mechanisms in vertical 2D semiconductor heterojunctions, offering a new way of engineering future two-dimensional materials based optoelectronic devices.

Utilizing aligned carbon nanotube arrays grown from chemical vapor deposition, we present a highly scalable route toward the formation of ribbons and ultrathin transparent films directly from vertically aligned single-walled carbon nanotube arrays (SWNT carpets). To "lay-over" the aligned nanotubes to form a film, we use a roller which acts to compress the film and preserve the alignment of nanotubes within the film. As we demonstrate, we can control the nanotube-catalyst interaction, leading to highly efficient transfer of the film to virtually any host substrate by following growth with a controlled H(2)O vapor etch. In addition, we demonstrate our ability to grow carpets on patterned substrates leading to upright carpet lines, which can be rolled over to form transparent films composed of ultralong carbon nanotubes. This work demonstrates a highly scalable technique to form homogeneous, transparent films of aligned SWNTs that can be ultralong with absolutely no need for liquid phase SWNT processing.

A scalable and facile approach is demonstrated where as-grown patterns of well-aligned structures composed of single-walled carbon nanotubes (SWNT) synthesized via water-assisted chemical vapor deposition (CVD) can be transferred, or printed, to any host surface in a single dry, room-temperature step using the growth substrate as a stamp. We demonstrate compatibility of this process with multiple transfers for large-scale device and specifically tailored pattern fabrication. Utilizing this transfer approach, anisotropic optical properties of the SWNT films are probed via polarized absorption, Raman, and photoluminescence spectroscopies. Using a simple model to describe optical transitions in the large SWNT species present in the aligned samples, polarized absorption data are demonstrated as an effective tool for accurate assignment of the diameter distribution from broad absorption features located in the infrared. This can be performed on either well-aligned samples or unaligned doped samples, allowing simple and rapid feedback of the SWNT diameter distribution that can be challenging and time-consuming to obtain in other optical methods. Furthermore, we discuss challenges in accurately characterizing alignment in structures of long versus short carbon nanotubes through optical techniques, where SWNT length makes a difference in the information obtained in such measurements. This work provides new insight to the efficient transfer and optical properties of an emerging class of long, large diameter SWNT species typically produced in the CVD process.

A controlled and scalable multistep purification method has been developed to remove iron impurity and nonnanotube carbon materials from raw single-walled carbon nanotubes (SWNTs) produced in the HiPco (high-pressure CO) process. In this study, iron nanoparticles, coated by carbon, are exposed and oxidized by multiple step oxidation at increasing temperatures. To avoid catalytic oxidation by iron oxide of carbon nanotubes, the exposed and oxidized iron oxide is deactivated by reaction with C 2 H 2 F 4 or SF 6 . The iron fluorides are removed by a Soxhlet extraction with a 6 M HCl solution. The purity and quality of each sample were determined by thermogravimetric analysis (TGA), Raman spectrometry, ultraviolet−visible-near-IR (UV−vis−near-IR) spectrometry, fluorescence spectrometry, and transmission electron microscope (TEM) spectroscopy. The purity and yield of SWNTs are improved due to reduced catalytic activity of the iron oxide. Greater iron oxide removal also resulted from oxidation at higher temperatures.In recent years, single-walled carbon nanotubes (SWNTs) have been intensively studied because of their many potential applications. The high-pressure CO (HiPco) process, where Fe(CO) 5 is used as catalyst, is one of the most productive methods for SWNT production. 1-3 However, the iron and nonnanotube carbon impurities in the produced material need to be removed without damaging the SWNT. To remove catalyst (typically iron, cobalt, and nickel) and obtain highpurity SWNTs, many purification methods have been reported previously. A common approach has been to use strong oxidation followed by an acid treatment. An oxidative treatment of raw SWNT material is effective in removing nonnanotube carbon and exposing the metal catalysts by removing carbon coating. However, nanotubes can be lost or damaged during the oxidation process. It is desirable that a scalable cleaning method only removes carbon impurities and metal catalysts without damaging nanotubes.In this study, we report a scalable multistep purification method to remove metal catalysts and remove nonnanotube carbon from raw HiPco SWNTs. Our scalable multistep purification method includes two processes: oxidation and deactivation of metal oxides. In the oxidation process, metal catalysts coated by nonnanotube carbon are oxidized by O 2 and exposed in multiple steps with increasing temperature steps (150°C through 350°C). In the deactivation step, exposed metal oxides are deactivated by conversion to metal fluorides through reacting with C 2 H 2 F 4 , SF 6 , or other fluorinecontaining gases to avoid the catalytic effect of iron oxide on SWNT oxidation. The proposed mechanism of purification is shown in Scheme 1. Figure 1 shows the thermal gravimetry analyses of raw HiPco SWNTs in N 2 /O 2 , N 2 /O 2 /C 2 H 2 F 4 , and N 2 /O 2 /SF 6 as function of time during heating. Weight gain during the initial heating is caused by the oxidation of iron particles. We found that all raw HiPco material is burned out at 325°C in air within less than 5 min witho...

An investigation into the optimal growth of single-walled carbon nanotubes (SWNTs) in vertical arrays, or carpets, is presented utilizing atomic hydrogen catalyst activation with hot filament chemical vapor deposition. Using acetylene decomposition over Fe catalyst, we study the effect of oxidant-assisted growth using O 2 , CO 2 , and H 2 O. Whereas trace amounts of O 2 result in the lack of any catalytic activity, CO 2 and H 2 O are found to dramatically enhance the catalyst lifetime. On the basis of the saturation effect of oxidant concentration for both CO 2 and H 2 O, we present this as being due to catalyst stabilization from surface hydroxyl groups, with H 2 O having the most dominant effect upon carpet growth. Utilizing water-assisted growth, this process is further optimized to yield high-quality single-walled carbon nanotubes. High temperature growth (∼775 °C) yields the highest-quality SWNTs, whereas controllable growth of double-and few-walled nanotubes can also be achieved at lower temperatures (550-600 °C). Finally, ultralong carpets are demonstrated by utilizing the optimal SWNT growth conditions under an enhanced carbon flux environment.

We use femtosecond optical pulses to induce, control and monitor magnetization precession in ferromagnetic Ga 0.965 Mn 0.035 As. At temperatures below ~40 K we observe coherent oscillations of the local Mn spins, triggered by an ultrafast photoinduced reorientation of the in-plane easy axis. The amplitude saturation of the oscillations above a certain pump intensity indicates that the easy axis remains unchanged above ~T C /2. We find that the observed magnetization precession damping (Gilbert damping) is strongly dependent on pump laser intensity, but largely independent on ambient temperature. We provide a physical interpretation of the observed light-induced collective Mn-spin relaxation and precession.The magnetic semiconductor GaMnAs has received considerable attention in recent years, largely because of its potential role in the development of spin-based devices 1,2 . In this itinerant ferromagnet, the collective magnetic order arises from the interaction between mobile valence band holes and localized Mn spins. Therefore, the magnetic properties are sensitive to external excitations that change the carrier density and distribution. Ultrafast pump-probe magneto-optical spectroscopy is an ideal technique for controlling and characterizing the magnetization dynamics in the magnetic materials, and has been applied to the GaMnAs system by several groups 3,4 .Although optically induced precessional motion of magnetization has been studied in 2 other magnetic systems 5 , magnetization precession in ferromagnetic GaMnAs has been observed only recently 4 and has yet to be adequately understood.In this paper, we report comprehensive temperature and photoexcitation intens ity dependent measurements of photoinduced magnetization precession in Ga 1-x Mn x As (x = 0.035) with no externally imposed magnetic field. By comparing and contrasting the temperature and intensity dependence of the precession frequency, damping, and amplitude, we identify the importance of light-induced nonlinear effects and obtain new information on the relevant physical mechanisms. Our measurements of the photoinduced magnetization show coherent oscillations, arising from the precession of collective Mn spins. Amplitude of the magnetization precession saturates above certain pump intensity is a strong indication that direction of the magnetic easy axis remains unchanged at temperatures above about half the Curie temperature (T C ). The precession is explained by invoking an ultrafast change in the orientation of the in-plane easy axis, due to an impulsive change in the magnetic anisotropy induced by the laser pulse. We also find that the Gilbert damping coefficient, which characterizes the Mn-spin relaxation, depends only weakly on the ambient temperature but changes dramatically with pump intensity. Our results suggest a general model for photoinduced precessional motion and relaxation of magnetization in the GaMnAs system under compressive strain.Time-resolved magneto-optical Kerr effect (MOKE) measurements were performed on a 300 nm thick f...

We report high-performance WSe phototransistors with two-dimensional (2D) contacts formed between degenerately p-doped WSe and undoped WSe channel. A photoresponsivity of ∼600 mA/W with a high external quantum efficiency up to 100% and a fast response time (both rise and decay times) shorter than 8 μs have been achieved concurrently. More importantly, our WSe phototransistor exhibits a high specific detectivity (∼10 Jones) in vacuum, comparable or higher than commercial Si- and InGaAs-based photodetectors. Further studies have shown that the high photoresponsivity and short response time of our WSe phototransistor are mainly attributed to the lack of Schottky-barriers between degenerately p-doped WSe source/drain contacts and undoped WSe channel, which can reduce the RC time constant and carrier transit time of a photodetector. Our experimental results provide an accessible strategy to achieve high-performance WSe phototransistor architectures by improving their electrical transport and photocurrent generation simultaneously, opening up new avenues for engineering future 2D optoelectronic devices.