Apparatus and methods are provided. A first apparatus includes: a semiconductor film; and at least one semiconductor nanostructure, including a heterojunction, configured to modulate the conductivity of the semiconductor film by causing photo-generated carriers to transfer into the semiconductor film from the at least one semiconductor nanostructure. A second apparatus includes: a semimetal film; and at least one semiconductor nanostructure, including a heterojunction, configured to generate carrier pairs in the semimetal film via resonant energy transfer, and configured to generate an external electric field for separating the generated carrier pairs in the semimetal film.

TECHNOLOGICAL FIELD

Embodiments of the present invention relate to photodetection. In particular, they relate to photodetection using semiconductor nanostructures.

BACKGROUND

A photodetector detects light by converting light incident upon it into electrical current.

BRIEF SUMMARY

According to various, but not necessarily all, embodiments of the invention there is provided an apparatus, comprising: a semiconductor film; and at least one semiconductor nanostructure comprising a heterojunction configured to modulate the conductivity of the semiconductor film by assisting photo-generated carriers to transfer into the semiconductor film from the at least one semiconductor nanostructure.

The semiconductor film may be graphene. The heterojunction may assist photo-generated carriers to transfer into the semiconductor film by separating them in the at least one semiconductor nanostructure. The heterojunction may be a type-II heterojunction.

The transfer of photo-generated carriers into the semiconductor film from the at least one semiconductor nanostructure may generate an electric field which modulates the conductivity of the semiconductor film.

The at least one semiconductor nanostructure may comprise first and second semiconductor nanomaterials that form the heterojunction.

The first and second semiconductor nanomaterials may be arranged relative to the semiconductor film such that photo-generated carriers are transferred from the second semiconductor nanomaterial to the semiconductor film, and not from the first semiconductor nanomaterial to the semiconductor film.

The second semiconductor nanomaterial may be in direct contact with the semiconductor film. The first semiconductor nanomaterial might not be in contact with the semiconductor film. The first semiconductor nanomaterial may be encased within the second semiconductor nanomaterial.

The apparatus may further comprise: an electrical bridge extending from the second semiconductor nanomaterial to the semiconductor film. The electrical bridge may be or comprise a metal. The metal may be or comprise one or more of: gold, platinum, palladium, nickel or copper.

According to various, but not necessarily all, embodiments of the invention there is provided a method, comprising: using a heterojunction of at least one semiconductor nanostructure to modulate the conductivity of a semiconductor film by assisting photo-generated carriers to transfer into the semiconductor film from the at least one semiconductor nanostructure.

The heterojunction may assist photo-generated carriers to transfer into the semiconductor film by separating them in the at least one semiconductor nanostructure. The heterojunction may be a type-II heterojunction.

The transfer of photo-generated carriers into the semiconductor film from the at least one semiconductor nanostructure may generate an electric field which modulates the conductivity of the semiconductor film.

According to various, but not necessarily all, embodiments of the invention there is provided an apparatus, comprising: a semimetal film; and at least one semiconductor nanostructure, comprising a heterojunction, configured to generate carrier pairs in the semimetal film via resonant energy transfer, and configured to generate an external electric field for separating the generated carrier pairs in the semimetal film.

The semimetal film may be graphene. The heterojunction may be a type-II heterojunction. The heterojunction may be configured to generate the external electric field by separating photo-generated carrier pairs. The carrier pairs in the semimetal may be generated via resonant energy transfer from photo-generated carrier pairs in the at least one semiconductor nanostructure.

The apparatus may further comprise a barrier between the at least one semiconductor nanostructure and the semiconductor film. The external electric field may be generated by separating the photo-generated carrier pairs bound within the at least one semiconductor nanostructure by the barrier. The external electric field may be approximately an electric dipole field. The at least one semiconductor nanostructure may be configured to generate the external electric field by assisting photo-generated carriers to transfer into the semimetal film from the at least one semiconductor nanostructure.

The at least one semiconductor nanostructure may comprise a first semiconductor nanomaterial and a second semiconductor nanomaterial that form the heterojunction. The heterojunction may extend in a direction that is substantially perpendicular to the directions of movement of the generated carrier pairs in the semimetal film.

The heterojunction may assist photo-generated carriers to transfer into the semiconductor film by separating them in the at least one semiconductor nanostructure.

The at least one semiconductor nanostructure may comprise first and second semiconductor nanomaterials that form the heterojunction. The first and second semiconductor nanomaterials may be arranged relative to the semiconductor film such that photo-generated carriers are transferred from the second semiconductor nanomaterial to the semiconductor film, and not from the first semiconductor nanomaterial to the semimetal film.

The second semiconductor nanomaterial may be in direct contact with the semimetal film. The first semiconductor nanomaterial might not be in contact with the semimetal film. The first semiconductor nanomaterial may be encased within the second semiconductor nanomaterial.

The apparatus may further comprise at least one further semiconductor nanostructure, comprising a further heterojunction, configured to generate further carrier pairs in the semimetal film via resonant energy transfer, and configured to generate a further external electric field for separating the further generated carrier pairs in the semimetal film.

The at least one semiconductor nanostructure may be configured to generate carrier pairs in the semimetal film via resonant energy transfer from photo-generated carrier pairs generated in the at least one semiconductor nanostructure in response to reception of light having a frequency equal to or above a first frequency threshold.

The at least one further semiconductor nanostructure may be configured to generate further carrier pairs in the semimetal film via resonant energy transfer from photo-generated carrier pairs generated in the at least one further semiconductor nanostructure in response to reception of light having a frequency equal to or above a second frequency threshold, which is lower than the first frequency threshold.

Current in the semimetal film may be produced in response to reception of light by the at least one semiconductor nanostructure and the at least one further semiconductor nanostructure having a frequency below the first frequency threshold and above or equal to the second frequency threshold, and current in the semimetal might not be produced in response to reception of light by the at least one semiconductor nanostructure and the at least one further semiconductor nanostructure having a frequency equal to or above the first frequency threshold and below the second frequency threshold.

According to various, but not necessarily all, embodiments of the invention there is provided a method, comprising: using at least one semiconductor nanostructure, comprising a heterojunction, to generate carrier pairs in a semimetal film via resonant energy transfer; and using the heterojunction to generate an external electric field for separating the generated carrier pairs in the semimetal film.

The external electric field may be generated by the heterojunction separating photo-generated carrier pairs.

DETAILED DESCRIPTION

Embodiments of the invention relate to photodetection using semiconductor nanostructures.

The figures illustrate an apparatus100/101, comprising: a semiconductor film10; and at least one semiconductor nanostructure20, comprising a heterojunction21, configured to modulate the conductivity of the semiconductor film10by assisting photo-generated carriers31to transfer into the semiconductor film10from the at least one semiconductor nanostructure20.

The figures also illustrate an apparatus102/103/104/105, comprising: a semimetal film11; and at least one semiconductor nanostructure20/120/20a-20e/20a-20j, comprising a heterojunction21/121, configured to generate carrier pairs230,131in the semimetal film11via resonant energy transfer, and configured to generate an external electric field60for separating the generated carrier pairs230,231in the semimetal film11.

FIG. 1illustrates a cross-section of first apparatus100comprising a semiconductor film10and a semiconductor nanostructure20. The semiconductor nanostructure20is located on the semiconductor film10between two electrical contacts2,3.

The example inFIG. 1showing a single semiconductor nanostructure20between the electrical contacts2,3is illustrative. It will be appreciated by those skilled in the art that, in practice, there may be multiple semiconductor nanostructures20located between the electrical contacts2,3.

In this example, the semiconductor film10is graphene. Graphene is both a zero-gap semiconductor and a semimetal. In other examples the semiconductor film10may, for example, be silicon.

The semiconductor nanostructure20comprises a first semiconductor nanomaterial22and a second semiconductor nanomaterial23that form a type-II (staggered gap) heterojunction21. In the illustrated example, the semiconductor nanostructure20is a nanorod. The first semiconductor nanomaterial22is encased (wholly) within the second semiconductor nanomaterial23.

The first semiconductor nanomaterial22is not in contact with the semiconductor film10inFIG. 1. It is separated from the semiconductor film10by the second semiconductor nanomaterial23.

The second semiconductor nanomaterial23is considered to be directly in contact with the semiconductor film10. A layer of extrinsic material might be located between the second semiconductor nanomaterial23and the semiconductor film10, the presence of which is difficult, inconvenient or impossible to avoid. The layer of extrinsic material may include the native oxide of the semiconductor nanostructure20, residual organic ligand from the nanostructure synthesis process, and/or absorbed water or molecules from the environment. The layer of extrinsic material is limited to a thickness of 5 nanometers (and preferably less).

In operation, when light is incident upon the semiconductor nanostructure20, carrier pairs (electron-hole pairs) are generated within the nanostructure20.FIG. 1illustrates an example in which an incident photon40generates a carrier pair30,31within the semiconductor nanostructure20. In theFIG. 1illustration, an electron31is shown in the second semiconductor nanomaterial23and a hole30is shown in the first semiconductor nanomaterial22.

The built-in field of the heterojunction21causes one type of carrier (electrons or holes)30,31to separate from the other. This prevents fast recombination in the nanostructure20and assists electrons or holes to transfer into the semiconductor film10by providing more time for tunneling to occur between the nanostructure20and the film10. In the example illustrated inFIG. 3, the electron31is illustrated as having tunneled into the semiconductor film10. The arrow labeled with the reference numeral37illustrates the transfer of the electron31into film10.

The transfer of a carrier31into the film generates an electric field between the nanostructure20and the film10, which modulates (increases or reduces) the conductivity of the film10. In this example, the conductivity of the film10is reduced when an electron31tunnels into the film10leaving a positively charged ion behind (as illustrated by the plus sign36inFIG. 3). The hole30does not transfer into the film10because it is positioned in the first semiconductor nanomaterial22which is separated from the film10by the second semiconductor material23. An electric field is therefore generated that is directed from the nanostructure20to the film10.

If a bias is applied across the film10using the contacts2,3, carriers that are transferred into the film10will flow across the film10between the contacts2,3.FIG. 3illustrates a situation in which the first contact2is positively charged relative to the second contact3, causing movement of the transferred electron31towards the first contact2, as illustrated by the arrow35.

FIG. 6illustrates a flow chart according to a first method. At block601inFIG. 6, a bias is applied across the semiconductor film10via the contacts2,3, prior to light being received by the semiconductor nanostructure20. In this example, the application of the bias results in current flowing from the first contact2to the second contact3.FIG. 2illustrates that, at this point in time, the conductivity of the film10is substantially constant.

At block602inFIG. 6, light is received at the nanostructure20, which results in charge transfer to the film10. The electric field that is generated from the charge transfer modulates the conductivity of the film10which, in this example, reduces the conductivity of the film10as shown in the graph illustrated inFIG. 4. The extent to which the conductivity of the film10is modulated depends upon the photon flux that is incident upon the semiconductor nanostructure20. The rate at which the conductivity of the film10is modulated depends upon the amount of extrinsic material located between the semiconductor nanostructure20and the semiconductor film10. A thinner extrinsic material barrier results in better carrier transfer into the film10, and a greater rate of change in conductivity.

In effect, the semiconductor nanostructure20acts as a photo-gate which is used to control the conductivity of a conductive channel, provided by the semiconductor film10, between the contacts2,3.

Advantageously, as described above, the heterojunction21in the semiconductor nanostructure20assists in the separation of photo-generated electrons and holes within the nanostructure20, preventing fast recombination. This provides more time for electrons or holes to transfer into the semiconductor film10and alter the conductivity of the film10.

In one example, the first semiconductor nanomaterial22may be cadmium sulfide (CdS), the second semiconductor nanomaterial23may be cadmium selenide (CdSe) and the semiconductor film10is graphene.FIG. 5illustrates an energy band diagram for such an example. An interface barrier12of extrinsic material is shown on the diagram. As mentioned above, the thickness of the interface barrier12is 5 nanometers or less.

FIGS. 7 to 11illustrate an embodiment of the invention which is similar to the embodiment illustrated inFIGS. 1 to 6and described above. A second apparatus101is illustrated inFIGS. 7 and 9which differs from the first apparatus100illustrated inFIGS. 1 and 3in that it further comprises an electrical bridge70. The electrical bridge70extends from the second semiconductor nanomaterial23to the semiconductor film10.

The electrical bridge70may be a metal. Preferably the metal used does not oxidize easily upon exposure to air. Metals such as gold, platinum, palladium, nickel and copper are suitable.

The electrical bridge70advantageously assists photo-generated carriers to transfer into the semiconductor film10from the nanostructure20. An arrow38inFIG. 9illustrates an electron31transferring into the electrical bridge70from the second semiconductor nanomaterial23. A further arrow39inFIG. 9illustrates the electron31transferring into the semiconductor film10from the electrical bridge70.

The graph inFIG. 8illustrates the conductivity of the semiconductor film10in this embodiment prior to light being received at the nanostructure. The graph inFIG. 10illustrates the decrease in the conductivity of the semiconductor film10when light is received at the nanostructure20. Advantageously, the rate at which the conductivity of the film10is modulated in this embodiment is greater than in the embodiment described above in relation toFIGS. 1 to 5, due to the presence of the electrical bridge70.

FIG. 11illustrates an energy band diagram for this embodiment of the invention, where the first semiconductor nanomaterial22is cadmium sulfide (CdS), the second semiconductor nanomaterial23is cadmium selenide (CdSe), the electrical bridge70is made from gold (Au) and the semiconductor film10is graphene. An epitaxial interface14is illustrated between the second semiconductor nanomaterial23and the electrical bridge70.

FIG. 12illustrates a cross-section of a third apparatus102which comprises a semimetal film11and a semiconductor nanostructure120. The semimetal film11may, for example, be graphene. In the illustrated example, the semiconductor nanostructure120is a nanorod.

The semiconductor nanostructure120is located on the semimetal film11between a first electrical contact2and a second electrical contact3. Unlike the embodiments described above in relation toFIGS. 1 to 11, in this embodiment a bias is not applied across the contacts2,3in operation.

The semiconductor nanostructure120is configured to generate carrier pairs230,231in the semimetal film11via resonant energy transfer, and configured to generate an external electric field60for separating the generated carrier pairs230,231in the semimetal film11. This is described in further detail below.

The semiconductor nanostructure120illustrated inFIG. 12comprises a first semiconductor nanomaterial122and a second semiconductor nanomaterial123which form a type-II (staggered gap) heterojunction121. In the illustration, the heterojunction121extends in a direction that is substantially perpendicular to the illustrated semimetal film11. In the illustrated example, the semiconductor nanostructure120is a nanorod. The illustrated heterojunction121passes through the center of nanostructure120and divides it in two.

The apparatus10further comprises a barrier (not shown inFIG. 12) that separates both the first semiconductor nanomaterial122and the second semiconductor nanomaterial123from the semimetal film11. The barrier may, for example, be 5 nanometers or greater in thickness and is intended to mitigate or prevent charge transfer between the semiconductor nanostructure120and the semimetal film11.

In operation, when light is incident upon the semiconductor nanostructure120(as illustrated by the photons40,41), carrier pairs (electron-hole pairs) are generated within the nanostructure120. The built-in field of the heterojunction121causes the photo-generated electrons to separate from the photo-generated holes.FIG. 12illustrates an example in which an incident photon40generates a carrier pair130,131within the semiconductor nanostructure120. In theFIG. 12illustration, the electron131is shown in the second semiconductor nanomaterial123and the hole130is shown in the first semiconductor nanomaterial122.

The photo-generated carrier pairs are bound within the semiconductor nanostructure120. The thickness of the barrier between the nanostructure120and the film11is such that the photo-generated carrier pairs cannot pass into the semimetal film11, or such that it is highly improbable that they will do so.

A photo-generated carrier (electron-hole) pair in the semiconductor nanostructure120can recombine without emitting a photon and instead exciting, via near field coupling, an equivalent carrier (electron-hole) pair in the semimetal film11. This is known as resonant energy transfer.

The separation of photo-generated electrons from photo-generated holes in the semiconductor nanostructure120generates an electric field60external to semiconductor nanostructure120. The electric field in this example is null within the nanostructure120and outside the nanostructure120the electric field60is approximately a dipole electric field, as illustrated inFIG. 12. The external electric field60is directed from the first semiconductor nanomaterial122to the second semiconductor nanomaterial123, as shown inFIG. 12by the plus and minus signs61,62.

The external electric field60separates the carrier pairs in the semimetal film11generated via resonant energy transfer.FIG. 12illustrates a hole231moving through the semimetal film11towards a first contact2under the influence of the external electric field60(see the arrow229).FIG. 12also illustrates an electron235moving through the semimetal film11towards a second contact3under the influence of the external electric field60(see the arrow235).

The movement of electrons and holes in the semimetal film11causes a current to be generated in the semimetal film11, as illustrated by the arrow75inFIG. 12. The direction of the current75therefore depends upon the direction of the electric field60generated by the semiconductor nanostructure120, which in turn depends upon the orientation of the heterojunction121in the nanostructure120. It can be seen inFIG. 12that the heterojunction121extends in a direction that is substantially perpendicular to the directions of movement of the carrier pairs in the film11(and is therefore perpendicular to the direction of current flow).

FIG. 13illustrates an fourth apparatus103which comprises multiple semiconductor nanostructures1200between the contacts2,3, where each semiconductor nanostructure has the same form as the semiconductor nanostructure120illustrated inFIG. 12and described above.

Each of the multiple semiconductor nanostructures1200is aligned with the same polarity inFIG. 13: the first semiconductor nanomaterial122in each nanostructure is closer to the first contact2than the second contact3, and the second semiconductor nanomaterial123in each nanostructure is closer to the second contact3than the first contact2. Each of the nanostructures inFIG. 13is arranged sufficiently far apart to avoid mutual electrical field compensation between adjacent nanostructures.

In operation, when light is incident upon the semiconductor nanostructures1200, a current75flows through the semimetal film11, from the first contact2to the second contact3.

Advantageously, the third and fourth apparatus102,103operate as a photodetector without the need for an electrical power input (due to absence of an applied bias across the contacts2,3).

The semiconductor nanostructures20a-20eare located on the semimetal film11between a first contact2and a second contact3. In this example, the semiconductor nanostructures20a-20eare positioned in a region that is closer to the second contact3than the first contact2. A bias is not applied across the contacts2,3in operation.

Each of the semiconductor nanostructures20a-20ehas the same form as the semiconductor nanostructure20illustrated inFIGS. 1 and 3orFIGS. 7 and 9. Each of the semiconductor nanostructures20a-20eis considered to be in contact with the semimetal film11. As in theFIGS. 1 to 11embodiments described above, there may be an interface layer12,14between the nanostructures20a-20eand the film11, or between the nanostructures20a-20eand the electrical bridge70(if any).

When light is received at the semiconductor nanostructures20a-20e, carrier pairs (electron-hole pairs) are generated within the nanostructures20a-20e. As described above in relation toFIGS. 1 to 11, the built-in field of the heterojunction21in each nanostructure20a-20ecauses one type of carrier (electrons or holes) to separate from the other. This prevents fast recombination in the nanostructure20a-20eand assists electrons or holes to transfer into the semimetal film11by providing more time for tunneling to occur between a nanostructure and the film11.

The transfer of carriers into the film11from the semiconductor nanostructures20a-20egenerates an electric field between the nanostructures20a-20eand the film11, which reduces the conductivity of the film11. In this example, electrons tunnel into the film11leaving positively charged ions, as illustrated by the plus sign36in each of the nanostructures20a-20einFIG. 14. This causes an external electric field to be generated that is directed from the nanostructures20a-20eto the film11. The presence of the electric field changes the energy band profile80of the semimetal film11, lowering it in the region where the semiconductor nanostructures20a-20eare positioned. The altered energy band profile80of the film11is illustrated by a dotted line inFIG. 14.

There is a maximum charge that each nanostructure20a-20ecan store. This can be considered to be the “saturation value”. Saturation occurs because, beyond a certain point, the external electric field that is generated between the nanostructures20a-20eand the film11hinders the transfer of the carriers between nanostructures20a-20eand the film11. A dynamic equilibrium will form between photo-generated carriers transferring from the nanostructures20a-20eto the film11and carriers transferring back from the film11to the nanostructures20-20edue to the external electric field.

Resonant energy transfer also takes place when light is received at the semiconductor nanostructures20a-20e. As explained above, in resonant energy transfer, photo-generated carrier (electron-hole) pairs recombine in the nanostructures20a-20ewithout emitting a photon and instead exciting, via near field coupling, an equivalent carrier (electron-hole) pair in the semimetal film11.

Once charge transfer between has taken place between the nanostructures20a-20eand the film11such that there is an electric field between them (and the energy band profile80of the film11has been adjusted), the movement of electrons and holes generated in the film11by resonant energy transfer is influenced by the electric field. This produces a current between the contacts2,3when light is incident upon the nanostructures20a-20e.

FIG. 14illustrates a hole229and an electron230being guided along the energy band profile80(see the arrows229and235). The direction of the current that is produced is from the second contact3towards to the first contact2.

Advantageously, the fifth apparatus104operates as a photodetector without the need for an electrical power input (due to the absence of an applied bias across the contacts2,3in operation).

FIG. 15illustrates a sixth apparatus105which comprises a semimetal film11, a first set of semiconductor nanostructures20a-20elocated on the semimetal film11and a second set of semiconductor nanostructures20f-20jlocated on the semimetal film11.FIG. 15also illustrates a series of photons40-43incident upon the apparatus105.

The first set and second sets of semiconductor nanostructures20a-20e,20f-20jare located on the semimetal film11between first and second contacts2,3, but the first set20a-20eis separated from the second set20f-20j. A bias is not applied across the contacts2,3in operation.

The form of the semiconductor nanostructures20a-20ein the first set is different from the form of the semiconductor nanostructures20f-20j; each of the semiconductor nanostructures20a-20ein the first set has a different band gap from the semiconductor nanostructures20f-20jin the second set.

The first set of semiconductor nanostructures20a-20emay, for example, be made from different material(s) from the second set of semiconductor nanostructures20f-20j.

Each of the semiconductor nanostructures20a-20ein the first set is sensitive to light having a frequency equal to or above a first threshold. That is, photo-generated carrier pairs are generated in the first set of semiconductor nanostructures20a-20ein response to reception of light having a frequency equal to or above the first threshold.

Each of the semiconductor nanostructures20f-20jin the second set is sensitive to light having a frequency equal to or above a second threshold. That is, photo-generated carrier pairs are generated in the second set of semiconductor nanostructures20f-20jin response to reception of light having a frequency equal to or above the second threshold. The second frequency threshold is lower than the first frequency threshold.

When light having a frequency lower than the both the first frequency threshold and the second frequency threshold is incident upon the first and second sets of semiconductor nanostructures20a-20j, no photo-generated carrier pairs are produced in the nanostructures20a-20jand therefore no current flows between the contacts2,3.

When light is incident upon the first and second sets of semiconductor nanostructures20a-20jwhich has a frequency equal to or higher than the first frequency threshold (and therefore also higher than the second frequency threshold) photo-generated carrier pairs are produced in both the first set and the second set of nanostructures20a-20e,20f-20j. The transfer of carriers between the first set of nanostructures20a-20eand the film11, and between the second set of nanostructures20f-20jand the film11, causes a change to the energy band profile81of the semimetal film11.FIG. 15illustrates the energy band profile of the semimetal film11in this situation.

The movement of carrier pairs that are generated in the film11via resonant energy transfer is influenced by the altered energy band profile81of the film11. Electrons generated in the film11from the first set of nanostructures20a-20eare guided towards the second contact3. Holes generated in the film11from the first set of nanostructures20a-20eare guided towards the first contact2. Conversely, electrons generated in the film11from the second set of nanostructures20f-20eare guided towards the first contact2. Holes generated in the film11from the second set of nanostructures20f-20jare guided towards the second contact3.

Consequently, the current that is generated by the first set of nanostructures20a-20eis cancelled out by the current that is generated by the second set of nanostructures20f-20j, so no overall current flows between the contacts2,3.

When light is incident upon the first and second sets of semiconductor nanostructures20a-20jwhich has a frequency below the first frequency threshold and above or equal to the second frequency threshold, photo-generated carrier pairs are produced in the second set of nanostructures20f-20jbut not the first set of nanostructures20a-20e. The transfer of carriers between the first set of nanostructures20a-20eand the film11causes a change to the energy band profile of the semimetal film11. However, there is no transfer of carriers between the first set of nanostructures20a-20eand the film11, so the energy band profile of the film11has the form of that illustrated inFIG. 14rather than that in illustrated inFIG. 15.

Carrier pairs are generated in the film11via resonant energy transfer, due to recombination of photo-generated electrons and holes in the first set of semiconductor nanostructures20a-20e. The movement of those carrier pairs is influenced by the altered energy band profile of the film11, producing a current in the film11.

In conclusion, the sixth apparatus105acts as a photodetector that is only sensitive to light having a frequency within a particular band of frequencies. Advantageously, the sixth apparatus105operates without an electrical power source (due to the absence of an applied bias across the contacts2,3in operation) and does not suffer from the losses introduced by optical filters in some photodetectors.

FIG. 16illustrates a second method according to embodiments of the invention. At block1601inFIG. 16, a semiconductor nanostructure20/120(as illustrated inFIG. 12,13,14or15), comprising a heterojunction21/121, generates carrier pairs230,231in a semimetal film11via resonant energy transfer. In block1602inFIG. 16, the semiconductor nanostructure20/120generates an external electric field for separating the generated carrier pairs230,231in the semimetal film11.

For instance, in some of the examples described above, charge transfer into the film10reduces the conductivity of the film10, but in other examples the conductivity of the film10may be increased.

It will be appreciated that carriers pairs may be generated in the film10via resonant energy transfer in embodiments of the invention described above in relation toFIGS. 1 to 11, but the photo-gating effect that occurs due to charge transfer between the nanostructure(s)20and the film10is orders of magnitude stronger than any direct photocurrent produced from resonant energy transfer.