Surface plasmons provide a means to enhance light-matter interaction through coherent electron oscillations that lead to strong electromagnetic fields at the surface of metals or any other material with a negative permittivity. In fact, the strong absorption and light scattering inherent in surface plasmon excitation has been used for thousands of years through the incorporation of metal particles in glass to create colorful stained glass windows and goblets . More recently, surface plasmon-based photonics, or “plasmonics” , , , has been used for enhancing light-matter interaction at the nanoscale in controlling light propagation , , emission , , and concentration , . Research on plasmonics has become a flourishing field with numerous applications , , including metamaterials and metasurfaces , , sub-diffraction-limited imaging , lasers , and cloaking . Plasmonics could also be used to enhance the performance of photovoltaic devices , , photodetectors and modulators , , and environmental sensors , .
Surface plasmons exist as either propagating surface plasmon polaritons (SPPs) on planar interfaces or localized surface plasmon resonances (LSPRs) that are confined to the surface of a particle. Surface plasmons are readily damped; thus, after a short time, the plasmon will start to decay radiatively, into reemitted photons , or nonradiatively , ,  via intraband or interband transitions, forming energetic or “hot” electrons. The plasmon decay processes after excitation are illustrated in Figure 1A. The hot electrons will further thermalize through electron-electron and electron-phonon coupling with the energy eventually being transferred to heat. In most cases, the nonradiative plasmon decay will serve as a parasitic process that limits the performance of plasmonic devices , for instance, limiting propagation length in plasmonic waveguides. In plasmonic metamaterials, the nonradiative decay process will lead to optical absorption in the metal and reduced performance. Although much effort has been devoted to mitigating plasmon nonradiative decay, recent research has uncovered exciting opportunities for harnessing the process , , such as in photothermal heat generation , , photovoltaic devices , , photocatalysis , , , driving material phase transitions , , photon energy conversion , and photodetection , , , , , , . For instance, the decay of hot electrons can lead to the local heating of the plasmonic nanostructures, making them candidates for nanoscale heat sources ,  for use in cancer therapy  and solar steam generation , . On the contrary, hot electrons can be captured before thermalization by an adjacent semiconductor, providing a novel photoelectrical energy conversion scheme for photovoltaics or for driving chemical reactions , . In this case, carriers excited with photon energies lower than the semiconductor bandgap can be captured, circumventing bandgap limitations and opening pathways for additional energy harvesting. This review focuses on the recent development of hot electron-based photodetection, and we point the reader to other review articles that cover chemical catalysis and photovoltaic applications , , . In this article, we will first cover the basic principles underlying hot carrier generation and extraction. Recent developments regarding hot electron-based photodetectors with varying device architectures will then be summarized. Finally, the current challenges and potential future directions will be discussed.
2 Hot carrier physics
2.1 Internal photoemission
Hot carriers generated in a metal can be collected by placing a semiconductor adjacent to the metal, forming a Schottky barrier. The hot carriers with energy greater than the Schottky barrier can flow into the semiconductor through an internal photoemission process, generating photocurrent (Figure 1B). The very first attempt to quantitatively describe the photon-induced emission of electrons from metals was made by Fowler  in the 1930s. Later in the 1950s, the overall hot electron generation and collection process was described by Spicer with an intuitive three-step model , : (1) hot electrons are generated in the metal through the absorption of photons, (2) a portion of the hot electrons diffuse to the metal-semiconductor interface before thermalization, and (3) hot electrons with sufficient energy and the correct momentum are injected into the conduction band of the semiconductor through internal photoemission. Therefore, the efficiency of hot carrier devices depends on the initial hot carrier distribution, the transport of the carriers, and the carrier collection efficiency. Fowler’s model is perhaps the most widely used description of hot electron injection and is based on a semiclassical model of hot electrons emitted over an energetic barrier, with the critical assumption that the photoexcited electrons in a metal have an isotropic momentum distribution and only the electrons within the momentum cone of the semiconductor can transport across the interface due to the conservation of electron momentum (Figure 1C). In Fowler’s model, the internal photoemission efficiency η is given by
where is the reduced Planck constant, ω is the photon frequency, EF is the metal Fermi energy, and ϕb is the barrier height. The barrier height is determined by the work function of the metal and the electron affinity of the semiconductor but can be affected by the fabrication process employed. The experimentally measured Schottky barrier heights for numerous metals and semiconductors are presented in Table 1.
In contrast to the electron-hole pair generation in semiconductors, the efficiency given by Eq. (1) predicts an increasing quantum yield at higher photon energy. Many recently demonstrated hot electron photodetectors based on Schottky barriers have been shown to follow this model , , . Compared to electron-hole generation and separation in a semiconductor, the internal photoemission over a bulk metal/semiconductor Schottky junction is a very inefficient process. This is mainly due to (1) poor light absorption, (2) a broad hot electron energy distribution , and (3) an isotropic hot electron momentum distribution. Surface plasmons can potentially boost the efficiency of hot electron collection by addressing these issues. Plasmonic metallic nanoparticles have an absorption cross-section much larger than the physical size of the particles , yielding much more efficient hot electron generation than bulk metal. Plasmonic nanoparticles also produce hot carriers at a higher average energy compared to absorption in bulk metals , , , . Lastly, the momentum distribution of hot carriers can be modified by engineering the modes of the plasmonic structures. Therefore, surface plasmons provide a powerful tool for efficient hot electron generation and extraction .
2.2 Hot carrier generation
Understanding initial hot electron generation and distribution, before inelastic relaxation, is critical to estimating the overall hot carrier injection efficiency. During nonradiative plasmon decay, hot electron-hole pairs are created through Landau damping on a time scale of femtoseconds . Theoretical calculations of hot electron energy distribution in bulk metal, based on the electron density of states approximated by a free electron gas model, shows a broad continuous distribution of hot electron energies upon excitation  (Figure 1B). This broad hot electron energy distribution sets an impediment for realizing high efficiency in the internal photoemission process, as many carriers will have an energy that is below the Schottky barrier . In contrast to bulk metals, it has been shown that the hot carrier energy distribution in plasmonic nanoparticles can potentially be narrowband , , , ,  and is strongly dependent on the particle size ,  and geometry , ,  (Figure 1D). For instance, hot carriers are more efficiently generated in small plasmonic nanoparticles compared to large ones . Nanoparticle geometry, including shape  and aspect ratio , also plays an important role in determining the hot carrier generation and injection due to carrier confinement and surface scattering . In addition, a theoretical model for the hot carrier generation process has been developed using Fermi’s golden rule, where the conduction electrons are described as free particles in a confined potential well . This model predicts that both the production rate and the energy distribution of the hot carriers are strongly dependent on the particle size and hot carrier lifetime. In addition to energy distribution, the hot carrier momentum distribution is also an important factor. Hot carriers are primarily generated with momentum parallel to the external field , which is generally parallel with the semiconductor interface in the case of an antenna, resulting in poor injection. Moreover, hot electron generation is also dependent on the plasmonic material employed , , , , . Several groups have explored the effects of the electronic structure of the metal on the generated carrier distributions ,  (Figure 1F), and it has been shown that, in the interband transition regime, the electronic band structure of the metal plays an important role in determining both the energy and the momentum distribution of the generated hot carriers . In fact, in this regime, none of the commonly used plasmonic materials, including gold, silver, aluminum, and copper, exhibit the isotropic momentum distribution assumed in Fowler’s theory . Additionally, the effects of phonons and surfaces on hot carriers have also been studied . Therefore, one needs to take plasmonic nanostructure geometry, size, excitation condition, hot carrier lifetime, and material electronic band structure into account when optimizing hot carrier device efficiency.
3 Photodetection with hot electrons
3.1 Free-space photodetectors
Early work on free-space photodetectors based on internal photoemission dates back almost half a century. In 1967, Peters demonstrated a photodetector using internal photoemission to detect low-energy, infrared photons . Later, free-space photodetectors based on this principle were developed for both Schottky ,  and MIM ,  diodes. Over the past few years, this area has seen renewed interest due to the advances in plasmonics, which provide a mechanism for strong light-matter interaction and enhanced resonant absorption and photoemission. One of the key features of hot electron injection is that it enables sub-bandgap photodetection. Therefore, hot electron photodetectors based on silicon , , , , , , , , , , ,  can be potentially employed in the telecommunications band, avoiding the need to use InGaAs and Ge detectors. In addition, the tunability of the plasmonic structures enables a straightforward means to tailor the response of a photodetector including the working wavelength, bandwidth, and polarization dependence , , , . For more information on general sub-bandgap photodetection we point the reader to a recent review on the topic .
In 2011, Knight et al. demonstrated a free-space hot electron photodetector based on a plasmonic nanoantenna and silicon, with the ability to detect near-infrared light below the silicon bandgap energy  (Figure 2A). In this device, the plasmonic nanoantenna serves simultaneously as the free-space light collector and electron emitter. After the nonradiative decay of surface plasmons, hot electrons are generated and injected into the conduction band of the n-type silicon substrate followed by the collection through an ohmic contact on the silicon. The top contact is formed by putting an indium tin oxide (ITO) conduction layer on top of the plasmonic antenna. By tuning the geometry of the plasmonic nanoantenna, one can control the absorption spectrum and therefore the photoresponsivity spectrum. Plasmonic gratings can also be used to realize selective, narrowband hot electron photodetectors  (Figure 2B). The extraordinary optical transmission observed in periodic gratings can strongly couple the free-space light into SPPs, resulting in a strong, narrowband resonant absorption. The electrically connected metallic grating structures also facilitate electron transport. Such gratings have been shown to possess a photoresponsivity of 0.6 mA/W and an internal quantum efficiency of 0.2%, a roughly 20 times improvement over the nanoantenna devices. A hot electron photodetector with further enhanced efficiency was demonstrated  (Figure 2C) by integrating the concept of metamaterial perfect absorbers ,  with the hot electron transfer process. The perfect absorption of light is realized by overlapping the LSPR with a Fabry-Perot resonance in the silicon cavity. Interestingly, this perfect absorption can be realized within a gold film of only 15 nm. This feature brings two advantages: (1) hot electrons are efficiently generated via the perfect absorption and (2) all the hot electrons are generated close to the Schottky interface, ensuring an efficient transport to the interface. This perfect absorber hot electron detector has been shown to exhibit a photoresponsivity of more than 3 mA/W while also preserving an omnidirectional response at a wavelength of about 1300 nm . The bandwidth can also be broadened by integrating multiple resonators . Pyramid nanostructures  have also been shown to enhance the photoresponsivity of hot electron detectors (Figure 2D). In this device, the silicon pyramids perform as efficient and broadband light concentrators, focusing the light from a large area into a small active pixel area where the hot electrons are generated. Metal layers are deposited on top of the silicon pyramids so that the Schottky interface is only formed at the apex region of the pyramids, ensuring a small dark current. More importantly, nanoscale confinement of plasmons at the apex of the tip plays a role in efficiency enhancement by relaxing momentum mismatch , . Operating at 0.1 V reverse bias, a responsivity of 5 and 12 mA/W at 1550 and 1300 nm, respectively, has been demonstrated. In addition, large area and low-cost fabrication techniques ,  can be used for silicon-based hot electron photodetectors. Hot electron photodetectors can be employed in the visible regime , , , ,  by swapping silicon with larger bandgap materials such as TiO2 , , , , ,  and ZnO . These detectors have much higher efficiencies than their telecommunication band counterparts due to the higher photon energy of visible light. Tandem-structured devices with double Schottky barriers can also be used to further enhance hot electron collection efficiency .
An alternative approach to extracting hot electrons is to form a metal-insulator-metal diode , , , , , , , . MIM diodes have been used as “rectennas”  at infrared and lower frequencies for power conversion and transmission. At optical frequencies, the operation of MIM diodes as rectification antennas has proven to be difficult , with efficiencies remaining low  due to the limitation on the RC time constant of the MIM diodes. However, the generation of hot electrons circumvents this limitation: hot electrons generated in one metal can tunnel through the thin insulating layer and be collected in the other metal contact, leading to a measurable photocurrent. Similar to the three-step model , , the photoemission of carriers across a MIM diode can be described in a series of five steps  as depicted in the band diagram shown in Figure 3A. This is similar to the junctions in plasmonic nanogap antennas , , where electrons can tunnel through the nanogap leading to photocurrent (Figure 3B). In the MIM diode, the hot electrons can be generated in both metal contacts; therefore, the key here is to create asymmetric hot electron generation or transport in the top and bottom metal layers. This can be done by creating asymmetric absorption, creating asymmetric barrier heights using different metals, or applying a bias to the device. In 2011, a hot electron-based MIM device  was proposed using an Au-Al2O3-Au diode for the application of solar energy conversion and photodetection (Figure 3C). In this planar unpatterned diode, a Kretschmann prism coupling configuration was used to couple the incident light to one of the electrodes, creating asymmetric absorption. This work predicted an optimized efficiency of 2.7% under AM 1.5 G illumination. The asymmetric hot electron generation in the top and bottom contacts can also be realized by creating LSPRs in one of the contacts , , , . It has been shown that asymmetric hot electron generation and enhancement of hot electron emission in Au-Al2O3-Au MIM diodes can be realized by reshaping the top metallic film contact into stripes  (Figure 3D). The stripes support an LSPR under TM illumination; therefore, the incoming light is mostly absorbed in the top contact. As a result, a measurable photocurrent is observed in the diode even without external bias. The stripe width-dependent and polarization-dependent photocurrent observed in this study further indicates that one needs to take the spatial and vectorial properties of the hot electron generation along with the material selection and diode geometry into account when designing the device.
Recently, researchers have explored new schemes to extract hot electrons for photodetection using transparent conducting oxides (TCO), including metal-insulator-TCO  and metal-semiconductor-TCO ,  junctions (Figure 3E and F). The use of TCO allows efficient electron collection while leading to asymmetric absorption and hot carrier generation yielding an efficient net photocurrent. In both schemes, light can be incident from the TCO side. Because the absorption of TCO is small, most of the incident light is absorbed in the metal layer and in the vicinity of the metal-insulator/semiconductor interface, leading to highly asymmetric and efficient hot carrier generation . Therefore, opposed to planar MIM diodes , special coupling is not necessary. The TCO-based structures have been demonstrated to produce responsivity up to 70 mA/W under a reverse bias of −3 V in the visible regime  (Figure 3F). In terms of detection speed, recent ultrafast studies on hot electron relaxation dynamics  have revealed hot electron relaxation times as fast as ~45 fs. Furthermore, the speed was strongly dependent on the local field enhancement and therefore can be controlled by tailoring the plasmonic geometry. These results indicate that the ultimate detection speed of hot electron devices could be extremely fast, although more work is still needed to understand the response time of a full device.
3.2 On-chip photodetectors
Hot electrons have also played a role in realizing on-chip photodetectors based on silicon with several demonstrations , , , , , , , ,  over the past several years. Despite the success of using silicon photonics for controlling light emission, propagation, and modulation, the use of silicon as an active absorbing material in the telecommunication band is challenging due its transparency in this regime. Efforts focused on developing complementary metal-oxide semiconductor (CMOS)-compatible, silicon-based on-chip photodetectors include using cavity-enhanced absorption , two-photon absorption (TPA) , and mid-bandgap generation . These approaches usually involve complicated fabrication processes or limit the operational bandwidth and power range. The use of hot electrons for photodetection can be employed by simply placing metal contacts on top of the silicon waveguide. The metal-silicon interface simultaneously serves as a Schottky interface for electrons and a metal-dielectric interface supporting SPP modes. A waveguide-based geometry can enable a higher internal quantum efficiency than free-space plasmonic antenna-based geometries due to two reasons. First, the SPPs propagate along the metal dielectric interface, naturally generating hot electrons in the vicinity of the Schottky interface. Second, in contrast to plasmonic antennas in which the electric field is primarily parallel to the Schottky interface, the SPPs have an electric field component perpendicular to the interface, therefore preferentially generating hot electrons with a momentum vector perpendicular to the Schottky interface.
Researchers have proposed ,  and experimentally demonstrated  waveguide-based Schottky detectors by putting metal stripes on silicon, forming SPP waveguides (Figure 4A). Internal responsivity of 0.38 and 1.04 mA/W were measured at a wavelength of 1280 nm  for gold and aluminum stripes on n-type silicon, respectively. A silicon waveguide-based SPP Schottky photodetector operating at telecom wavelengths has also been developed  (Figure 4B). The device was fabricated using a self-aligned approach of local oxidation of silicon (LOCOS) on a silicon on insulator substrate (Figure 4B). Internal responsivity of 0.25 and 13.3 mA/W were measured for wavelengths of 1.55 and 1.31 μm under a reverse bias of 0.1 V . Later, an internal responsivity of 12.5 mA/W at a wavelength of 1.55 μm  was demonstrated by optimizing the device design (Figure 4C). Other than improving the photoresponsivity, there are also efforts focused on improving the detection speed of on-chip hot electron photodetectors , ,  by minimizing the junction capacitances of the photodetector. This is realized by reducing the metal-semiconductor contact area by directly depositing the metal contact on the vertical output facet of the waveguide (Figure 4D). With this detection scheme, a bandwidth up to 1 GHz has been achieved while also maintaining a responsivity of 3.5 mA/W at a negative bias of −21 V . Recently, the bandwidth is further improved and a hot electron photodetector with data reception rate of 40 Gbit/s has been demonstrated . A Schottky detector has also been applied to Si-cored fiber  for applications such as power monitoring in optical fiber communication. Other than using Schottky interfaces on silicon waveguides, MIM diodes have also been used in a waveguide geometry  (Figure 4E). MIM diodes can be built on a wider range of devices such as polymer waveguides and optical fiber. In addition, multiple metal contacts , , ,  are not required here, simplifying the geometry , , although quantum efficiency suffers when compared to a Schottky diode.
3.3 Hot electrons with 2D materials
Over the past decade, the unique properties of graphene and other 2D materials, such as transition metal dichalcogenides, have made them attractive for optoelectronic and photonic applications , , . The high electron mobility, broadband absorption, ultrafast response, and electrical tunability of these 2D material systems enables their application in ultrafast and ultrasensitive light detection , . However, due to their atomic thickness, the light-matter interaction in 2D materials is usually limited. On the contrary, plasmonics enable strong light-matter interaction with 2D materials due to high field concentration , , , . Additionally, it has been shown that surface plasmons in metallic nanoparticles can directly interact with 2D materials through hot electron injection , , , , , . For instance, hot electrons can change the doping of graphene  or molybdenum disulfide (MoS2) , , leading to structural phase transitions  or modulation of the absorption spectrum . Hot electrons can also be used for enhancing MoS2 photocatalysis of hydrogen evolution  and the photoluminescence of MoS2 . A photodetector based on hot electron injection into graphene has also been reported  (Figure 5A). In this device, nanoscale plasmonic antennas are sandwiched between two graphene monolayers. Surface plasmons enhance the photocurrent in two ways: by transferring hot electrons generated from plasmon decay in the antenna and by enhancing the near-field and direct electron-hole pair generation in graphene , . The contribution of direct photoexcitation and hot carrier injection was also investigated by femtosecond pump-probe measurements . In addition, MoS2 has also been used for extracting hot electrons generated from plasmonic nanostrucutres , , . Photodetectors working in the near-infrared regime with photoresponsivity on the level of A/W have been demonstrated (Figure 5B) by combining photogain in bilayer MoS2 with hot electron injection, although the large responsivity is accompanied by a relatively slow response time . Further research in this direction is needed as novel functionalities and unprecedented performance could potentially be realized by combining hot electron injection with new 2D materials such as InSe  and WS2 .
3.4 Novel functionalities
Research in the emerging field of hot electron photodetection has not only focused on improving the efficiency but also on exploring novel functionalities, which do not exist in conventional semiconductor photodetectors. For instance, nanoscale surface imaging can be realized using the hot electron injection from a plasmonic nanotaper (Figure 6A). A gold nanotaper adiabatically compresses the SPPs at the end of the tip before injecting the electrons into the semiconductor substrate. By scanning the plasmonic nanotaper on the semiconductor, the local chemical sensitivity of the semiconductor surface can be obtained from the hot electron photocurrent map (Figure 6A). In addition, one can use the tunability of plasmonic structures to tailor the response of the photodetector including the working wavelength, bandwidth, and polarization dependence , , , , , . Recently, a circularly polarized light detector was demonstrated by combining a chiral metamaterial with hot electron injection  (Figure 6B). The chiral metamaterial can perfectly absorb circularly polarized light with one particular handedness while largely reflecting the complimentary component. Therefore, it can selectively generate hot electrons and produce a photocurrent signal depending on the handedness of light . This ultracompact detector avoids the complexity of conventional circularly polarized light detection schemes, where a quarter wave-plate and polarizer are used. Another example of a hot electron photodetector with tailored responsivity is a narrowband photodetector  (Figure 6C). The narrowband responsivity comes from the narrowband absorption of the plasmonic grating that supports extraordinary optical transmission. One can also easily tune the responsivity peak position by changing the interslit distance. This narrowband photodetector could find applications in biomedical sensing, imaging, and surveillance where a small wavelength bandwidth is needed, whereas other bands must be suppressed. Another way to perform wavelength-sensitive detection is using the MIM diode (Figure 6D). It has been demonstrated that MIM diodes can be used for direct wavelength determination and real-time deconvolution of multiples signals at different wavelengths based on electron tunneling . In this case, the open-circuit voltage between the upper and lower electrodes in the MIM diode is directly related to the incident photon energy and is not dependent on incident power. This detector provides simplicity compared to grating-based spectroscopy and could potentially lead to the realization of on-chip spectrometry. With the compactness, tunability, and novel functionalities of hot electron detectors, we believe that hot electron photodetectors enabling the full characterization of light (polarization, wavelength, and wave vector) could be realized in a highly integrated platform.
4 Future developments and perspectives
Research on plasmon-induced hot electron injection has experienced much progress over the past several years. However, an improvement in device performance is needed to address practical applications. Progress will require improving the efficiency of hot carrier generation and the subsequent transport and injection processes. This involves both the judicious design of plasmonic nanostructures and a better fundamental understanding of the hot carrier generation, transport, and extraction processes.
In terms of plasmonic design, ways to further enhance the field concentration and absorption in ultrathin or small nanoparticles is needed , , , . In terms of hot carrier transport, employing higher-quality noble metals ,  or exploring new plasmonic materials , , , ,  could potentially result in longer hot carrier diffusion lengths. Hot electrons can also experience significant reflection as a result of the large wave-vector contrast at the metal-dielectric interface. As such, more work is needed in the design of plasmonic nanostructures that support the generation of hot electrons with a momentum distribution that matches well with the band structure of the semiconductor. For instance, the recent investigation that uses plasmonic tips to adiabatically compress the SPPs, producing hot electrons with momentum perpendicular to the interface, demonstrated a greatly enhanced efficiency . Hot electron transfer efficiencies can further be boosted by embedding the plasmonic nanostructures within the semiconductor, providing more momentum space for hot electron emission  (Figure 7A). In addition, it has been shown that the preferred carrier type for extraction, either electrons or holes, is dependent on the plasmonic material employed , .
A more fundamental understanding of hot carrier generation, transport, and emission is still needed for designing efficient hot carrier devices. For hot carrier distribution, although much work has been devoted to providing theoretical predictions of hot carrier distribution in plasmonic nanostructures , , , , , , more experimental work is needed to validate these models. In addition, a better understanding of the hot carrier relaxation timescale , , , ,  in various materials as well as the transport dynamics in nanostructures would provide more valuable information for designing plasmonic devices with efficient hot carrier transport to the Schottky interface. Lastly, engineering the metal-semiconductor interface on the atomic level could also lead to improved hot electron transfer efficiencies , . For instance, a recent report describing hot electron transfer by a plasmon-induced interfacial charge-transfer transition has shown an internal quantum efficiency up to 20% independent of incident photon energy .
In conclusion, we have reviewed the recent developments in the field of hot electron-based photodetection ranging from fundamental studies regarding hot carrier generation and extraction to hot electron devices for both free-space and on-chip photodetection. Although we have focused on the hot electron-based photodetection, the basic principles and designs presented here can be applied to any system where charge separation is involved, such as photovoltaics , , , photocatalysis , , , as well as light emission  and modulation . There are many open challenges in the field such as establishing a better understanding of the carrier injection process, characterizing device speed, and exploring a wider range of plasmonic materials. Ultimately, we believe that advances in these areas should lead to a wealth of opportunities and scientific breakthroughs in the future.
W.L. and J.G.V. were supported by the National Science Foundation (NSF) under the program CBET-1336455.
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About the article
Published Online: 2016-11-01
Competing financial interests: The authors declare no competing financial interests.
Citation Information: Nanophotonics, Volume 6, Issue 1, Pages 177–191, ISSN (Online) 2192-8614, ISSN (Print) 2192-8606, DOI: https://doi.org/10.1515/nanoph-2015-0154.
©2016, Jason G. Valentine et al., published by De Gruyter.. This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 3.0 License. BY-NC-ND 3.0