news & views OPTOELECTRONIC DEVICES
Monolayer diodes light up p–n diodes can be fabricated from a single layer of WSe2 crystal.
Rudolf Bratschitsch
S
eventy-five years ago, Russell Ohl of Bell Labs in the US accidentally discovered the p–n junction whilst trying to purify silicon crystals1. Today, p–n junctions are essential elements in several devices including diodes and solar cells. Such junctions are boundaries between two regions of a semiconductor — termed p (positive) and n (negative) — that have different levels of impurity atoms. Recently, atomically thin transition-metal dichalcogenides, such as MoS2, MoSe2, WS2 and WSe2, have attracted considerable interest because of their promising electrical and optical properties2. These two-dimensional materials are similar to graphene, but differ in one important aspect — they are direct-gap semiconductors. Up to now, MoS2 was the most studied transition-metal dichalcogenide, but it has proven difficult to obtain hole conduction in this monolayer material3, hindering fabrication of an atomically thin p–n junction. Writing in Nature Nanotechnology, three independent research teams now show that a monolayer of WSe2 can be used to create p–n diodes4–6. The three teams — who are led by Thomas Mueller 4 at the Vienna University of Technology, Pablo Jarillo-Herrero5 at the Massachusetts Institute of Technology, and Xiaodong Xu6 at the University of Washington — all used the same design concept of a lateral diode, in which the p–n junction is created electrostatically within a WSe2 monolayer by means of two independent gate voltages applied to adjacent regions of the layer (Fig. 1). The diodes exhibit ambipolar transport: that is, both electrons and holes are injected from different sides into the channel area. In contrast to chemical doping, electrostatic doping allows these WSe2 diodes to be operated either as a p–n or n–p diode, depending on the applied gate voltages. In this way, the devices are able to rectify current flowing in either direction. When applying gate voltages of same polarity to both regions of the monolayer (in p–p or n–n configurations), the devices act as simple resistors. Besides a display of rectifying current– voltage characteristics, the monolayer p–n
Photoconductor
Photodiode
Solar cell
Light-emitting diode
Source
Gate 1
WSe2
Drain
Insulator SiO2
Gate 2
Si
Figure 1 | Schematic drawing of a lateral monolayer WSe2 p–n diode with split-gate electrodes. The metal back gates are separated from the WSe2 monolayer by an insulator, which can be SiN, HfO2 or BN. Blue and orange spheres represent electrons and holes, respectively.
diodes fabricated by the three teams can also be used as different types of optoelectronic device. In particular, they can operate as a photovoltaic solar cell, a photoconductor, a photodiode or a light-emitting diode. The solar cells have a power-conversion efficiency — which measures the percentage of incident energy of light converted into electrical energy — of less than 1% (ref. 4). This value is relatively high, if you keep in mind that the atomically thin WSe2 layer is almost entirely transparent, so that only a few per cent of the incident photons are absorbed. When operated as a resistor, the device behaves as a photoconductor, but its slow response time and large power consumption render it impractical for real-world applications. The WSe2 monolayer diodes, however, perform better when operated as photodiodes, showing a responsivity to light detection of hundreds of mA W–1 (refs 4,5). Furthermore, the versatility of the monolayer p–n diodes allows them to be operated as electrically driven light emitters, with bright electroluminescence observed at room temperature4–6. In this set-up,
NATURE NANOTECHNOLOGY | VOL 9 | APRIL 2014 | www.nature.com/naturenanotechnology
© 2014 Macmillan Publishers Limited. All rights reserved
electrons and holes are electrically injected on either side of the WSe2 monolayer and the devices emit light when the electrons and holes recombine in the centre (Fig. 1). Spatial mapping of the electroluminescence (electrically driven light emission) shows that light originates from the entire length of the monolayer junction between the two gates6. The electroluminescence and the photoluminescence (optically driven light emission) spectra are almost identical, suggesting that the same physical mechanism is at work for both excitations. No electroluminescence is detected in n–n or p–p configurations, confirming that light emission is generated exclusively by the p–n junction. The driving current of monolayer WSe2 p–n diodes is of the order of a few nanoamperes, which compares favourably with the much higher current levels in light-emitting MoS2 field-effect transistors. However, the electroluminescence efficiency — the optical output power divided by electrical input power — is still low, and less than 1%. Resistive losses at the contacts and non-radiative recombination in 247
news & views the atomically thin WSe2 sheet are likely to be the factors limiting this value4. There is scope for improving the performance of these monolayer WSe2 diodes by optimizing the fabrication methods. The WSe2 monolayers were prepared by mechanical exfoliation4–6 and transfer techniques involving polymers. Mechanically exfoliated flakes have sizes of only a few micrometres, and the usefulness of these monolayer diodes for lighting and transparent display technologies will depend on the ability to produce these materials with large areas. The growth of transition-metal dichalcogenide
monolayers under controlled conditions, together with more sophisticated fabrication techniques, will significantly improve the electrical and optical properties of the p–n diodes. Better performance would also be expected by shifting from a lateral to a vertical diode design, where the monolayer materials are stacked on top of each other 7. In this way the area of the junction is significantly increased. Finally, the possibility of optically accessing the valley degree of freedom of carriers, instead of using their charge or spin, by circularly polarized light2 could lead to the design of devices with novel functionalities. ❐
Rudolf Bratschitsch is at the Institute of Physics, University of Münster, 48149 Münster, Germany. e-mail: [email protected] References 1. Riordan, M. & Hoddeson, L. Crystal Fire: The Birth of the Information Age (W. W. Norton, 1997). 2. Jariwala, D., Sangwan, V. K., Lauhon, L. J., Marks, T. J. & Hersam, M. C. ACS Nano 8, 1102–1120 (2014). 3. Zhang, Y. J., Ye, J. T., Yomogida, Y., Takenobu, T. & Iwasa, Y. Nano Lett. 13, 3023–3028 (2013). 4. Pospischil, A., Furchi, M. M. & Mueller, T. et al. Nature Nanotech. 9, 257–261 (2014). 5. Baugher, B. W. H., Churchill, H. O. H., Yang, Y. & Jarillo-Herrero, P. Nature Nanotech. 9, 262–267 (2014). 6. Ross, J. S. et al. Nature Nanotech. 9, 268–272 (2014). 7. Geim, A. K. & Grigorieva, I. V. Nature 499, 419–425 (2013).
ULTRASOUND IMAGING
Better contrast with vesicles Biologically derived, nanoscale gas vesicles can be used as ultrasound contrast agents.
Mark Borden and Shashank Sirsi
U
ltrasound is widely used in hospitals and clinics as a medical imaging tool, but it trails other techniques, such as magnetic resonance imaging and optical imaging, when it comes to measuring and mapping biological events in the body at the molecular level. The reason for this is simple: ultrasound employs mechanical waves rather than electromagnetic waves. A molecule must mechanically vibrate or ‘ring’ to be detected by ultrasound, and although biological molecules can absorb and emit photons, they cannot ring due to their highly viscous surroundings1. Even if the biomolecules could ring, their response would be at acoustic frequencies that are too high to be detected with current ultrasound technology. To achieve molecular imaging with ultrasound, a probe with a large mechanical response is required, and over the past 30 years researchers have focused on the use of lipid- or protein-stabilized gas microbubbles as probes. In one such approach, molecules are detected by monitoring the change in ultrasound scattering intensity as microbubble probes, which are covered in ligands, bind to biological receptors and accumulate in the target tissue2. These microbubble probes are prefabricated and then injected intravenously, where they bind to receptors on cells that line the surface of blood vessels. Owing to their size, however, microbubbles cannot typically pass through the lining of the blood vessels (the endothelium) to probe extravascular structures, and this inability 248
to get beyond the vasculature limits the utility of ultrasound for molecular imaging. In contrast, other imaging approaches, such as magnetic resonance imaging and optical imaging, have been able to use ligandcoated nanoparticles that are small enough to diffuse through the endothelium and be internalized by cells. Writing in Nature Nanotechnology, Mikhail Shapiro and co-workers3 at the University of California at Berkeley and the University of Toronto now show that biologically derived gas vesicles can provide ultrasound contrast. The gas vesicles are nanoscale biconical cylinders with diameters of 45–250 nm and lengths of 100–600 nm, which are formed by certain prokaryotic cells in lakes and oceans to regulate their buoyancy for harvesting light. The gas vesicles have a thin (~2 nm) protein shell that is hydrophobic on the inner surface and hydrophilic on the outer surface4. The crystalline shell provides structural rigidity to maintain the hollow core of the gas vesicle, allowing dissolved gases to diffuse inside while liquid water is excluded. The size and shape of the gas vesicles are genetically encoded in the prokaryotic DNA and their assembly is tightly regulated. The gas vesicles can, therefore, be produced biologically with excellent compositional and structural precision. The researchers first demonstrate that the gas vesicles are highly echogenic: that is, they are good scatterers of ultrasound in the clinically relevant frequency range (4.8–17 MHz). In their study, they
differentiate highly scattering gas vesicles (those produced by the cyanobacteria Anabeana flox-aquea) from those that are highly absorbing (those produced by the archaea Halobacterium NRC-1); the highly scattering gas vesicles being preferred for ultrasound contrast. They then show that gas vesicles can emit acoustic harmonics, which are critical for improving the sensitivity of ultrasound detection. These in vitro studies are then complemented by a series of in vivo studies that show prolonged ultrasound contrast following subcutaneous and intravenous injection in mice. A gas vesicle has a volume that is around 103-fold smaller than a typical microbubble, which considerably expands the potential range of ultrasound molecular imaging to extravascular and intracellular targets. Molecular imaging of the protein avidin with biotinylated gas vesicles was demonstrated by a change in the acoustic scattering intensity with avidin concentration. The biotin–avidin–biotin crosslinks caused the gas vesicles to cluster, which provided enhanced ultrasound backscatter. One potential molecular imaging application for this approach could be extravascular imaging, where individual gas vesicles are designed to travel from the vessels into tissue and form clusters on interacting with the target molecule (Fig. 1a). Shapiro and colleagues also demonstrate that the acoustic scattering signal from gas vesicles can be silenced by using ultrasound amplitudes exceeding the critical buckling pressure of the gas vesicles. The buckling
NATURE NANOTECHNOLOGY | VOL 9 | APRIL 2014 | www.nature.com/naturenanotechnology
© 2014 Macmillan Publishers Limited. All rights reserved
Monolayer diodes light up p–n diodes can be fabricated from a single layer of WSe2 crystal.
Rudolf Bratschitsch
S
eventy-five years ago, Russell Ohl of Bell Labs in the US accidentally discovered the p–n junction whilst trying to purify silicon crystals1. Today, p–n junctions are essential elements in several devices including diodes and solar cells. Such junctions are boundaries between two regions of a semiconductor — termed p (positive) and n (negative) — that have different levels of impurity atoms. Recently, atomically thin transition-metal dichalcogenides, such as MoS2, MoSe2, WS2 and WSe2, have attracted considerable interest because of their promising electrical and optical properties2. These two-dimensional materials are similar to graphene, but differ in one important aspect — they are direct-gap semiconductors. Up to now, MoS2 was the most studied transition-metal dichalcogenide, but it has proven difficult to obtain hole conduction in this monolayer material3, hindering fabrication of an atomically thin p–n junction. Writing in Nature Nanotechnology, three independent research teams now show that a monolayer of WSe2 can be used to create p–n diodes4–6. The three teams — who are led by Thomas Mueller 4 at the Vienna University of Technology, Pablo Jarillo-Herrero5 at the Massachusetts Institute of Technology, and Xiaodong Xu6 at the University of Washington — all used the same design concept of a lateral diode, in which the p–n junction is created electrostatically within a WSe2 monolayer by means of two independent gate voltages applied to adjacent regions of the layer (Fig. 1). The diodes exhibit ambipolar transport: that is, both electrons and holes are injected from different sides into the channel area. In contrast to chemical doping, electrostatic doping allows these WSe2 diodes to be operated either as a p–n or n–p diode, depending on the applied gate voltages. In this way, the devices are able to rectify current flowing in either direction. When applying gate voltages of same polarity to both regions of the monolayer (in p–p or n–n configurations), the devices act as simple resistors. Besides a display of rectifying current– voltage characteristics, the monolayer p–n
Photoconductor
Photodiode
Solar cell
Light-emitting diode
Source
Gate 1
WSe2
Drain
Insulator SiO2
Gate 2
Si
Figure 1 | Schematic drawing of a lateral monolayer WSe2 p–n diode with split-gate electrodes. The metal back gates are separated from the WSe2 monolayer by an insulator, which can be SiN, HfO2 or BN. Blue and orange spheres represent electrons and holes, respectively.
diodes fabricated by the three teams can also be used as different types of optoelectronic device. In particular, they can operate as a photovoltaic solar cell, a photoconductor, a photodiode or a light-emitting diode. The solar cells have a power-conversion efficiency — which measures the percentage of incident energy of light converted into electrical energy — of less than 1% (ref. 4). This value is relatively high, if you keep in mind that the atomically thin WSe2 layer is almost entirely transparent, so that only a few per cent of the incident photons are absorbed. When operated as a resistor, the device behaves as a photoconductor, but its slow response time and large power consumption render it impractical for real-world applications. The WSe2 monolayer diodes, however, perform better when operated as photodiodes, showing a responsivity to light detection of hundreds of mA W–1 (refs 4,5). Furthermore, the versatility of the monolayer p–n diodes allows them to be operated as electrically driven light emitters, with bright electroluminescence observed at room temperature4–6. In this set-up,
NATURE NANOTECHNOLOGY | VOL 9 | APRIL 2014 | www.nature.com/naturenanotechnology
© 2014 Macmillan Publishers Limited. All rights reserved
electrons and holes are electrically injected on either side of the WSe2 monolayer and the devices emit light when the electrons and holes recombine in the centre (Fig. 1). Spatial mapping of the electroluminescence (electrically driven light emission) shows that light originates from the entire length of the monolayer junction between the two gates6. The electroluminescence and the photoluminescence (optically driven light emission) spectra are almost identical, suggesting that the same physical mechanism is at work for both excitations. No electroluminescence is detected in n–n or p–p configurations, confirming that light emission is generated exclusively by the p–n junction. The driving current of monolayer WSe2 p–n diodes is of the order of a few nanoamperes, which compares favourably with the much higher current levels in light-emitting MoS2 field-effect transistors. However, the electroluminescence efficiency — the optical output power divided by electrical input power — is still low, and less than 1%. Resistive losses at the contacts and non-radiative recombination in 247
news & views the atomically thin WSe2 sheet are likely to be the factors limiting this value4. There is scope for improving the performance of these monolayer WSe2 diodes by optimizing the fabrication methods. The WSe2 monolayers were prepared by mechanical exfoliation4–6 and transfer techniques involving polymers. Mechanically exfoliated flakes have sizes of only a few micrometres, and the usefulness of these monolayer diodes for lighting and transparent display technologies will depend on the ability to produce these materials with large areas. The growth of transition-metal dichalcogenide
monolayers under controlled conditions, together with more sophisticated fabrication techniques, will significantly improve the electrical and optical properties of the p–n diodes. Better performance would also be expected by shifting from a lateral to a vertical diode design, where the monolayer materials are stacked on top of each other 7. In this way the area of the junction is significantly increased. Finally, the possibility of optically accessing the valley degree of freedom of carriers, instead of using their charge or spin, by circularly polarized light2 could lead to the design of devices with novel functionalities. ❐
Rudolf Bratschitsch is at the Institute of Physics, University of Münster, 48149 Münster, Germany. e-mail: [email protected] References 1. Riordan, M. & Hoddeson, L. Crystal Fire: The Birth of the Information Age (W. W. Norton, 1997). 2. Jariwala, D., Sangwan, V. K., Lauhon, L. J., Marks, T. J. & Hersam, M. C. ACS Nano 8, 1102–1120 (2014). 3. Zhang, Y. J., Ye, J. T., Yomogida, Y., Takenobu, T. & Iwasa, Y. Nano Lett. 13, 3023–3028 (2013). 4. Pospischil, A., Furchi, M. M. & Mueller, T. et al. Nature Nanotech. 9, 257–261 (2014). 5. Baugher, B. W. H., Churchill, H. O. H., Yang, Y. & Jarillo-Herrero, P. Nature Nanotech. 9, 262–267 (2014). 6. Ross, J. S. et al. Nature Nanotech. 9, 268–272 (2014). 7. Geim, A. K. & Grigorieva, I. V. Nature 499, 419–425 (2013).
ULTRASOUND IMAGING
Better contrast with vesicles Biologically derived, nanoscale gas vesicles can be used as ultrasound contrast agents.
Mark Borden and Shashank Sirsi
U
ltrasound is widely used in hospitals and clinics as a medical imaging tool, but it trails other techniques, such as magnetic resonance imaging and optical imaging, when it comes to measuring and mapping biological events in the body at the molecular level. The reason for this is simple: ultrasound employs mechanical waves rather than electromagnetic waves. A molecule must mechanically vibrate or ‘ring’ to be detected by ultrasound, and although biological molecules can absorb and emit photons, they cannot ring due to their highly viscous surroundings1. Even if the biomolecules could ring, their response would be at acoustic frequencies that are too high to be detected with current ultrasound technology. To achieve molecular imaging with ultrasound, a probe with a large mechanical response is required, and over the past 30 years researchers have focused on the use of lipid- or protein-stabilized gas microbubbles as probes. In one such approach, molecules are detected by monitoring the change in ultrasound scattering intensity as microbubble probes, which are covered in ligands, bind to biological receptors and accumulate in the target tissue2. These microbubble probes are prefabricated and then injected intravenously, where they bind to receptors on cells that line the surface of blood vessels. Owing to their size, however, microbubbles cannot typically pass through the lining of the blood vessels (the endothelium) to probe extravascular structures, and this inability 248
to get beyond the vasculature limits the utility of ultrasound for molecular imaging. In contrast, other imaging approaches, such as magnetic resonance imaging and optical imaging, have been able to use ligandcoated nanoparticles that are small enough to diffuse through the endothelium and be internalized by cells. Writing in Nature Nanotechnology, Mikhail Shapiro and co-workers3 at the University of California at Berkeley and the University of Toronto now show that biologically derived gas vesicles can provide ultrasound contrast. The gas vesicles are nanoscale biconical cylinders with diameters of 45–250 nm and lengths of 100–600 nm, which are formed by certain prokaryotic cells in lakes and oceans to regulate their buoyancy for harvesting light. The gas vesicles have a thin (~2 nm) protein shell that is hydrophobic on the inner surface and hydrophilic on the outer surface4. The crystalline shell provides structural rigidity to maintain the hollow core of the gas vesicle, allowing dissolved gases to diffuse inside while liquid water is excluded. The size and shape of the gas vesicles are genetically encoded in the prokaryotic DNA and their assembly is tightly regulated. The gas vesicles can, therefore, be produced biologically with excellent compositional and structural precision. The researchers first demonstrate that the gas vesicles are highly echogenic: that is, they are good scatterers of ultrasound in the clinically relevant frequency range (4.8–17 MHz). In their study, they
differentiate highly scattering gas vesicles (those produced by the cyanobacteria Anabeana flox-aquea) from those that are highly absorbing (those produced by the archaea Halobacterium NRC-1); the highly scattering gas vesicles being preferred for ultrasound contrast. They then show that gas vesicles can emit acoustic harmonics, which are critical for improving the sensitivity of ultrasound detection. These in vitro studies are then complemented by a series of in vivo studies that show prolonged ultrasound contrast following subcutaneous and intravenous injection in mice. A gas vesicle has a volume that is around 103-fold smaller than a typical microbubble, which considerably expands the potential range of ultrasound molecular imaging to extravascular and intracellular targets. Molecular imaging of the protein avidin with biotinylated gas vesicles was demonstrated by a change in the acoustic scattering intensity with avidin concentration. The biotin–avidin–biotin crosslinks caused the gas vesicles to cluster, which provided enhanced ultrasound backscatter. One potential molecular imaging application for this approach could be extravascular imaging, where individual gas vesicles are designed to travel from the vessels into tissue and form clusters on interacting with the target molecule (Fig. 1a). Shapiro and colleagues also demonstrate that the acoustic scattering signal from gas vesicles can be silenced by using ultrasound amplitudes exceeding the critical buckling pressure of the gas vesicles. The buckling
NATURE NANOTECHNOLOGY | VOL 9 | APRIL 2014 | www.nature.com/naturenanotechnology
© 2014 Macmillan Publishers Limited. All rights reserved
