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Sathish Chander Dhanabalan1, Joice Sophia Ponraj2, Qiaoliang Bao2§, and Han Zhang1# 1

SZU-NUS Collaborative Innovation Center for Optoelectronic Science and Technology, and Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province, Shenzhen University, Shenzhen, China, 518060 2 Institute of Functional Nano and Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials and Devices, and Collaborative Innovation Center of Suzhou Nano Science and Technology, College of Physics and Microelectronic Science, Soochow University, Suzhou, China, 215123 §

#

: [email protected], : [email protected]

Abstract Recent research on photo-detectors has been mainly focused on nanostructured materials that form the building blocks of device fabrication. The selection of suitable material with welldefined properties forms the key issue for the fabrication of photodetectors that cover different range of the electromagnetic spectrum. In this review, the latest progress in light detection by using nanobelts, nanoribbons, nanosheets and the emerging two-dimensional (2D) materials is reviewed. Particular emphasis is placed on the light detection by using the hybrid structures of those mentioned nanostructured materials in order to enhance the efficiency of light-matter interaction. Other rising direction like black phosphorus based photo-detection is also reviewed. This review may provide an overview of some basic concepts and new directions in the photo-detectors, and highlight some potential for the future development of high performance broadband photo-detector.

Keywords: Photodetectors, Nanobelts, Nanoribbons, Nanosheets, Two-dimensional materials

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Present perspectives of broadband photo-detectors based on nanobelts, nanoribbons, nanosheets and the emerging 2D materials

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1. Introduction In the development of optoelectronics, the realization of peculiar photodetectors plays a

field.

1-5

The basic prerequisites of photodetectors (PDs) have been well described in few

pioneering works based on their knowledge at that period of time which could be revealed based on Olympic principles such as Citius, Altius, Fortius corresponding to the photodetectors principles of faster, more sensitive, smarter, respectively.

6

The term was

proposed by G. van Aller and S-O. Flyckt of Philips Laboratories in the year 1983.

7

Photodetectors are significant optoelectronic devices engaged in conversion of light to electricity by means of photoelectric effect. photoconductors and photodiodes.

9

8

Photodetectors can be broadly classified as

Photoconductors represent the conductivity change in

materials that are placed between two electrodes. Photodiodes deal with the behaviour dependence in p-n or Schottky junctions under illumination of light.

2, 10

The external

quantum efficiency (EQE) of photodiodes is usually less than 1 attributed to the insufficient absorption of light and carrier recombination. Moreover, they have fast response caused by electrons and holes associated to the photocurrent generation which recombine once they reach their own electrodes leading to short carrier lifetime. In the case of photoconductor, only one type of carrier is responsible for photocurrent while another one is trapped in the photoconductor.

9

Photodetectors may find widespread applications in detection (radiation,

smoke, flame, missile plume), engine monitoring, switching relays for street lights, atomic force microscopy, laser based devices (laser guided missiles, laser warning, security systems, laser range finders, alignment and control systems), communication (optical free-air, lightwave, intersatellite), spectral analysis in medical field, chemical/biological sensors, sensors in space applications (solar, star), missile launch, spectral monitoring of earth’s ozone layer and automotive anti-collision optical radar.

11-22

Owing to the photoconductive properties of

semiconducting materials, they form the basic element of the photodetector. 5, 23 Nanostructures can be broadly classified in different categories such as zero-dimensional (quantum dots), one-dimensional (nanowires, nanorods, nanotubes, nanobelts, nanoribbon) and two-dimensional (nanosheets, monolayers, ultra-thin film). Due to the quantum confinement effect and dimensionality dependence, semiconducting nanostructures were found to exhibit unique size-dependent physical properties that vary significantly with respect to the corresponding bulk materials. 24, 25 The difference in dimensionality leads to the diverse bandgap energies of nanocrystals owing to the confinement in various dimensionalities and

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major role as building blocks in providing solutions for emerging problems in this research

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bandgap structure is an important factor for quantitative modification by controlling the size, dimensionality and composition.

26-29

The optical and electrical properties vary with size of

systematic transformations in the density of electronic energy levels with a function of the size, otherwise called as quantum size effects.

24, 30

Photodetectors fabricated from one-

dimensional (1D) structures display high sensitivity and high quantum efficiency in contrast to their bulk analogues by virtue of its increased surface-to-volume ratio, Debye length comparable to their small size and reduced dimensionality.

31-36

On the other hand, the

increase in surface-to-volume ratio is significant in designing miniaturized portable devices and also provides greater advantage for detection sites for sensing. With the launching of considerable gap states in large bandgap 1D nanostructures, photoresponse spectral region can be significantly broadened and this could be an alternative to overcome the disadvantage of 1D based PDs of large bandgap operating under light with narrow spectral range.

15, 32, 37

Owing to the strong dependence of nanomaterials’ bandgap on their sizes (smaller than few nanometres comparable to Bohr radius) which allows the spectral tunability within a single material system, making the way for multispectral photodetectors that are sensitive in the visible, near-infrared (NIR) and ultraviolet (UV).

2

Moreover, this can also extend the

photoactive region towards visible or even infrared region. It thereby incomparably enhances the performance of the optoelectronic devices and photocatalysts.

38-41

The photocarrier

lifetime can be further enhanced, which is attributed to the charge separation and migration developed by surface states. 42, 43 The reduced dimensionality significantly plays a significant role in the confinement of active area of the charge carriers leading to shorter transit time. 35 An interesting fact is that these 2D nanostructures can be more compatible with the current thin film micro-manufacturing techniques along with their easy fabrication into complex structures in contrast with quantum dots and other 1D materials (nanowires, nanorods and carbon nanotubes).

44

These nanostructures could be assembled in the fabrication of

photodetectors owing to their dimensionality, compact size, ease of manipulation, precise crystal structures, rationally designed surface and peculiar 1D enclosing surfaces.

16, 23, 45

Furthermore, the response speed of the photodetectors is influenced by the size of nanoribbons.

46

In the year 2001, Wang and co-workers

47

first reported nanobelts as 1D

structurally controlled nanomaterial with well-defined chemical composition, crystallography and enclosing surfaces. The nanobelts/nanoribbons have been adopted in the fabrication of nanometer-scale multiband light sensors due to their large surface area exposed to the light.

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semiconductor nanocrystals having the same interior bonding geometry. This is caused by the

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46, 48-53

The electronic and optoelectronic device performances are improved because

nanobelts can be nearly void of dislocations and line defects. Single crystalline nanobelts

ribbon (belt)-like morphology make them suitable for the development of miniaturized device applications and fabrication of nanoscale functional devices. 54 The individual nanobelts have been demonstrated to inherit interesting optoelectronic properties. 55 Being compared to the photodetectors fabricated from bulk or thin film counterparts, some novel properties and characteristics such as strong polarization dependence and better photoresponse of ultra-thin two dimensional (2D) nanostructures have drawn much attention. 44, 46

. Hence, this review is concentrated on nanosheets (NS), nanobelts (NB) and nanoribbons

(NR) with the two main aspects of different wavelength ranges and the materials adopted for the demonstration of the device structures. The intention of this review is to supplement detailed knowledge of broadband, ultraviolet, visible and infrared photodetectors based on the two-dimensional materials and nanostructures in the form of nanobelts, nanoribbons and nanosheets. The first part of this article presents some important parameters of photodetectors. The next part deals with different spectra of photodetectors based on the above-mentioned nanostructures with some peculiarities. Various types of photodetectors will simultaneously be described. Finally, the outlook will discuss the current challenges photodetectors are now facing in terms of materials selection, functionality and fabrication that has to be taken into account for future advancement that are necessary for further development in the field of photodetectors of 2D nanostructures in fabrication and processing. It is anticipated that this review can provide an overview and evaluation of stateof-the-art broadband photodetectors based on nanobelts, nanoribbons, nanosheets and the emerging two-dimensional material. 2. Important Parameters of Photodetectors: Device

mechanisms:

The

significant

characteristics

of

desired

high-performance

photodetectors in practical applications include high photosensitivity, high stability, high spectral selectivity, fast response speed, high gain and small device size. 56-58 Photocurrent (∆I) is defined as, ∆I=Ip - Id

………. (1)

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with specific oriented surfaces having rectangular cross section, uniform thickness, and

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where Ip, Id represent the current at bias voltage of 0.1 V in the dark and under light illumination, respectively.

significant parameters namely, spectra responsivity (Rλ) and external quantum efficiency. The spectra responsivity, also called as current responsivity is defined as the photocurrent generated per unit power of incident light on the effective area of a photodetector. The external quantum efficiency is the number of electrons detected per incident photon. The large values of Rλ and EQE suggest high sensitivity for photodetectors and are expressed as follows: 23, 59 Rλ= ∆Iλ/(PλS)

………. (2)

EQE = hcRλ/eλ

………. (3)

in which ∆Iλ is the photocurrent induced by the incident light of wavelength (λ), Pλ is the light intensity, S is the effective illuminated area, h, c, e represent the Plank constant, velocity of light and charge of electron, respectively. The major parameters involved in photodetection mechanism of nanostructured (NS/NB/NR) photodetectors in wide-range of electromagnetic spectrum from previous reports are tabulated in Table-1. Peicai et al. 60 described three main optoelectronic processes in semiconductor nanostructured photodetectors. They are (1) photodoping effect: resulting from the upliftment of quasi-Fermi level of the carriers originated by photogenerated excess carriers thereby causing increase in conductivity

46, 59, 61

(2) photoconductance effect: arising from band bending achieved by

surface electric field at surface of nanostructures 14, 62, 63 and (3) photogating effect: trapping of photogenerated excess minority carriers that would extend the photocarrier lifetime. 15, 46, 61 3. Different Spectra of Photodetectors Graphene (G) with its unique optical, electrical, magnetic, optical and mechanical properties is a booming research area mostly reported in recent days.

64-68

Graphene is an interesting

candidate in optoelectronic devices with stimulating properties of high response speed, broad spectral detection width, wavelength independent absorption and high carrier mobility.

69-74

The unfavourable condition is that its gapless nature cause low responsivity and low photoconductive gain which could be a major problem in photoconductors.

4, 75, 76

The

limitation of graphene arises from its zero bandgap which cause a serious issue in different

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The quantitative and qualitative performance of photodetectors are determined by the two

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applications such as photodetectors or photoswitching microdevices as they demand definite ON and OFF states. -2

64, 77, 78

The demerits of graphene-based photodetectors are low

-1

selectivity.

79, 80

Researchers have suggested that the integration of graphene with other 2D

materials to form van der Waals heterostructure might be a fascinating process to overcome the above-mentioned issues. In view of this, photodetectors based on MoS2-graphene, WSe2graphene

heterojunction,

graphene-Bi2Te3 heterostructure

have

attained

enhanced

performance with high responsivity. 81-84 Therefore, 2D materials came into the picture to add flavours to the photodetectors and hybrid photodetectors based on graphene/2D materials has turned into a contemporary leader in photodetectors. We will review in detail the photodetectors fabricated from nanobelts, nanoribbons and nanosheets to be operated in different spectral ranges of incident light in electromagnetic spectrum. 3.1 Broadband Photodetectors The two dimensional materials-based photodetectors are found to have demonstrated the ability to detect light over a broad spectral range with a few limitations related to polarization sensitivity. The small and direct bandgap of black phosphorus (BP) makes it appealing for broadband photodetection in which transition metal dichalcogenides (TMDCs) are limited owing to their large bandgap. 86-89 Bulk BP shows high carrier mobility (~ 10000 cm2/Vs) and a ~0.3 eV direct bandgap whereas its bandgap increases with the decrease in thickness and is predicted to exhibit direct bandgap of >1 eV in its monolayer form.

90-92

The layered BP

crystal with a rectangular in-plane lattice exhibit a highly-anisotropic structure along the two perpendicular x and y directions in which every two rows of P atoms form an “armchair”-like geometry by preferably puckering up and down to only along the x direction. 94

93

Gomez et al

reported that transmission electron microscopy measurements confirm the crystallinity and

stability of exfoliated freely suspended flakes. This is due to the fact that the electrons and photons in BP behave in a highly anisotropic manner within the layer plane.

93, 95-97

Moreover, the orthorhombic layered structure together with layer-dependent direct bandgap from monolayer to bulk also favours BP in photo-detection. BP is an excellent candidate for incorporating intrinsic crystal anisotropy in polarization-sensitive photo-detection compared to that of conventional photodetectors available for linear dichorism detection reliable on extrinsic geometric effects.

98-102

It is interesting to compare layered BP having highly-

anisotropic structure with rectangular in-plane lattice to other 2D materials such as graphene

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responsivity (~10 AW ), low external quantum efficiency (0.1-0.2%) and lack of spectral

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and molybdenum sulphide (MoS2) having hexagonal in-plane lattice that are insensitive to the linear polarization of incident light. 98

with rise time of about 1 ms and their suitability for broadband and fast detection was therefore understood.

86

It was demonstrated that a gate tunable p-n diode based on a p-type

BP/n-type monolayer MoS2 van der Waals p-n heterojunction being a promising material for broadband photodetection and solar energy harvesting. Figure 1 (a-c) show the schematics of the device formed by p+ silicon wafer capped with 285 nm SiO2 acted as global back gate and the gate dielectric is also clearly depicted. A van der Waals heterojunction was designed with few tens of layered BP flakes exfoliated onto monolayer MoS2 and Ni/Au were later deposited to serve as contacts. The electrical measurements were made by applying a voltage Vd across the device and bias voltage Vg was applied to the back gate. Linear-dichroic broadband photodetector with layered BP transistors as seen in Figure 2 (a-c) are reported by employing the strong intrinsic linear dichroism originating from the in-plane optical anisotropy with respect to the atom-buckled direction and this device is polarization sensitive over a broad bandwidth from 400 nm to 3750 nm.

98

Linear dichroism probes

different absorption of light polarized parallel or perpendicular to an orientation axis which directly depends on both the conformation and orientation whether device material/device is intrinsically oriented in an anisotropic crystal structure or extrinsically oriented in anisotropic patterns. 12, 100, 103 The important aspect is that the perpendicular built-in electric field induced by gating in black phosphorus transistors could spatially separate the photo-generated electrons and holes in the channel thereby reducing their recombination rate with enhancement of efficiency and performance for linear dichroism photo-detection. Polarization-sensitive broadband photodetector using a black phosphorus vertical p–n junction is also recently reported. 99 The photoresponse of field-effect transistors (FETs) made of few tens of layered BP (3- 8 nm thick) as a function of excitation wavelength, power and frequency was studied by Buscema et al.

86

. Figure 2 (d-f) depicts the optical image, device schematics and photocurrent

behaviour of few tens of layered BP FETs. The optical image, photocurrent mapping and photodetection responsivity of BP/MoS2 heterojunction is given in Figure 2 (g-i). It showed a maximum photodetection responsivity of 418 mA/W (633 nm wavelength) and photovoltaic energy conversion with an external quantum efficiency of 0.3%. 90 The samples used in their

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The BP transistors showed response to excitation wavelengths from the visible to 940 nm

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study have thicknesses of the few tens of layered black phosphorus flake and monolayer MoS2 as ~11 nm and ~0.9 nm, respectively. The increase in the back gate voltage resulted in resistance and the MoS2/metal contact resistance. The photoresponsivity of these devices is nearly 100 times higher than that of the recently reported black phosphorus phototransistor and 4.8 times more than the carbon nanotube-MoS2 p-n diode for smaller voltage applied across the diode. 86, 104 Owing to the exotic properties of 2D nanostructured materials of high surface-volume ratio and reduced dimensionality, they became potential candidate in nanophotonic device components. Molybdenum trioxide (MoO3), an intrinsic n-type II-VI semiconductor with wide bandgap (~3.2 eV), finds application in organic electronics as efficient anode interfacial layers attributed to its high work function. 32 Among metal oxide nanostructures, Xiang et al. 32

reported photodetectors based on MoO3 nanobelt irrespective of their low conductivity and

weak photoresponsivity

85

which was enhanced by the introduction of substantial gap states

by means of H2 annealing. Their goal was to enhance the conductance significantly with enrichment of gap states in MoO3 annealing in order to attain excellent photoresponse in the wide visible spectra region. Figure 3 (a-c) shows the scanning electron microscopy (SEM) image and schematic illustration of device for photocurrent measurement along with the time dependent photoresponse before and after annealing under 660 nm laser illumination. MoS2 is a semiconductor having a bandgap range from 1.2 eV to 1.8 eV as the thickness decreases from bulk to monolayer in which the bandgap depends on the number of layers. 105107

Being a direct bandgap semiconductor which permits high absorption coefficient and

electron-hole pair generation rate during photoexcitation, monolayer MoS2 finds application in optoelectronic device circuits, light sensing, biomedical imaging, video recording and spectroscopy.

107-111

Field effect transistors based on monolayer or multilayer MoS2 possess

high current ON/OFF ratios of up to 107-108.

112, 113

MoS2 photodetector with a

poly(vinylidene fluoride-trifluoroethylene) ferroelectric layer instead of the oxide layer in a traditional field effect transistor was realized as seen in Figure 3 (d-f). The ferroelectric polarization suppressed the dark current of the photodetector. 114 Interestingly, the bandgap of few-layer MoS2 could be altered by the ultra-high electrostatic field from the ferroelectric polarization. Therefore, photoresponse wavelengths of the photodetector were broadened into the near infrared (0.85-1.55 µm). They suggested that the ferroelectrics/optoelectronics

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considerable increase in photocurrent and this is caused by the reduction of the MoS2 sheet

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hybrid structure would be an effective way to accomplish the high performance 2D electronic/optoelectronic devices.

layered CdI2-type structure where the tin atoms are positioned between two hexagonal closepacked sulfur slabs to form a three-atom sandwich structure (S–Sn–S triple layer) so that it can be exfoliated to form two-dimensional crystals. 116 Sn and S are bound by strong covalent force within each triple layer and triple layers are bound by van der Waals interactions. 117, 118 Recently, a new flexible and rigid broadband photodetector based on SnS2nanosheet selfassembled microsphere film was devised onto a transparent polypropylene film using doubleside adhesive tape.

119

The reported detector demonstrated excellent photoresponse in the

regime of 300 to 830 nm with outstanding photo-switching effect and stability. It was found that both the flexible and rigid photodetectors had same photosensitive properties and photosensitive mechanism. Bi2Te3 belongs to topological insulator family with a similar hexagonal symmetry that of graphene. Graphene-Bi2Te3 photodetector had shown enhanced photoresponsivity (35 AW-1 at a wavelength of 532 nm) and sensitivity (photoconductive gain up to 83) with respect to pure graphene based devices. The resultant device exhibited increased photocurrent attributed to the effective photocarrier generation and transfer at the interface between graphene and Bi2Te3.

84

The SEM image of Bi2Te3 nanoplatelets on graphene, schematic of the fabricated

photodetector device and photocurrent as a function of gate voltage are given in Figure 3 (gi). They had also reported that the wavelength limit could be extended to near-infrared (980 nm) and telecommunication band (1550 nm). Feng et al. 4

-1

12

115

realized ultrahigh

13

photoresponsivity (~ 10 AW ) and detectivity (~ 10 to 10 Jones) that can be adjustable using applied gate voltage in multilayer indium selenide (InSe) nanosheets phototransistors with broadband response from UV to near infrared wavelength. Figure 3 (j-l) portrays the schematic drawing of InSe nanosheet phototransistor, schematic of the photon absorption of InSe multilayer and the band diagram of InSe nanosheet phototransistor. It was reported that multiple reflection interference at the interfaces of the device leads to thickness-dependent photo-response. 3.2 Ultraviolet Photodetectors The monitoring of UV rays is demanding due to the major health issue of skin cancer caused by the radiations coming to the earth. The bandgap of zinc sulphide (ZnS) is 3.72 and 3.77 eV

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The semiconducting material tin disulfide (SnS2) with a direct band gap of ~ 2.2 eV has

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for cubic zinc blend and hexagonal wurtzite forms respectively. The potential of ZnS as a UV detector in this specific wavelength regime is higher in comparison with the available

semiconductor zinc oxide, ZnO (~3.4 eV).

120-122

The photoresponsivity of ZnS-nanobelt-

based UV-light sensors was found to have over three orders of magnitude gain under UVlight illumination than that of visible light. 49 Figure 4 (a-c) exhibits the transmission electron microscopy (TEM) image of ZnS nanobelts (Inset: SEM image of a single-crystalline ZnSnanobelt device), schematic diagram of device and the time response upon 320 nm light illumination with and without UV light, respectively. The high spectral selectivity, high photosensitivity and fast response time (<0.3 s) make the single-crystalline ZnS nanobelts attractive for new ‘‘visible-light-blind’’ UV-light photodetectors, particularly in the UV-A region. UV nano-photodetector based on chloride doped zinc sulphide (Cl:ZnS) nanoribbon/Au Schottky junctions with photoresponsivity of 9.1 AW-1 was also reported. 123 Due to the low electrical performances of UV photodetectors fabricated from solely inorganic semiconductors, the blooming research area namely the hybridization is employed recently to improve the photocurrent thereby eliminating the above-mentioned limitations. The high electrical conductivity, enhanced mechanical flexibility, impermeability (to standard gases), 124-126

transparency of graphene makes it interesting to be incorporated into photoactive

semiconductors for the enhancement of light absorption and carrier transportation.

127

There

were few studies adopting the hybrid structures of graphene and semiconductors showed improved UV photodetection with few limitations of small area of effective-junction region attributing to the photocurrent.

128-130

Kim et al.

127

realized solution-grown ZnS nanobelts

and CVD-grown graphene based high-performance hybrid structured UV photodetectors of both sandwiched and multilayer stacked structures with excellent spectral selectivity and stable ON-OFF switching. They have used three different UV photodetectors in ambient conditions at a low bias voltage: (i) ZnS spin-coated on the surface of double-layer graphene, (ii) ZnS sandwiched between two graphene layers (S-G/ZnS) and (iii) multiple sandwiched graphene and ZnS (MS-G/ZnS). Interestingly, the photo-response behaviour of a photodetector was influenced by both the number of layers and the stacking sequence of graphene and ZnS. In comparison with the previous literature UV photodetectors based on ZnS nanobelts

49

and ZnS-ZnO nanowires

131

without graphene, optimized photodetector

based on S-G/ZnS exhibited photocurrent (37 µA) that is 106 times greater than the reported values. Figure 4 (d-f) shows the TEM image of S-G/ZnS nanobelts, schematic diagram of

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substitute materials such as indirect band-gap diamond (~ 5.5 eV) and direct bandgap

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device and energy level diagram of S-G/ZnS heterojunctions presenting charge-transfer process under UV-light illumination, respectively. MS-G/ZnS photodetector showed

photodetector based on ZnS nanobelts.

49

The photodetection mechanism of these devices

was explained based on the spontaneous charge transfer of photoexcited electrons in the conduction band of ZnS to graphene channels. 127 ZnS/ZnO biaxial nanobelts having diameters in the range of few tens of nm to 100 nm and lengths up to tens of µm have been exploited for UV-A photodetector. 36 It had high spectral selectivity and wide-range photoresponse and was found to be much better than that of pure ZnS or ZnO nanostructures. The mismatch stress between the ZnS and ZnO lattices cause the bending or biaxial nanobelts to be curved of ZnS/ZnO biaxial nanobelt.

132

Hu et al.

36

reported an interesting mechanism of photodetection in the ZnS/ZnO biaxial nanobelt. The SEM image of ZnS/ZnO nanobelts (Inset: High-resolution SEM image) and device structure are given in Figure 4 (g,h). Interestingly, the illumination of light leads to the electron-hole pairs generation and the internal field at the ZnS/ZnO interface makes the electrons move towards the ZnO and holes move towards the ZnS. It forms a charge transfer state and spatial separation of photogenerated carriers within the nanobelts which in turn decreases the electron-hole pair recombination thereby increasing the photocurrent and EQE, see Figure 4 (i). The high photocurrent might be attributed to the Cr/Au electrodes and good ohmic contact without any interfacial barrier or traps between the ZnS/ZnOnanobelt. Self-powered ultraviolet photodetector based on a single antimony doped zinc oxide (Sb:ZnO) nanobelt bridging an Ohmic contact and a Schottky contact was demonstrated with high photoresponse sensitivity ( 2200%) and the response time (<100 ms). 133 They have found the degradation in performance of their device with the decrease in the concentration of antimony-doping in the ZnO nanobelt. In this case, energy of the photon from UV is higher than the bandgap of ZnO than the bandgap of ZnO focussed towards the NB creates the photo-generation of electronhole pairs. Gallium sulphide (GaS) is a member of III−VI group where the interlayer interactions are dictated by the weak van der Waals force and interlayer-bonding forces are covalent in nature. It has an indirect band gap of 2.59 eV at 300 K, a direct band gap at ~3.05 eV and therefore used in near-blue light emitting devices. Highly crystalline, exfoliated GaS nanosheets based photodetectors were demonstrated on both mechanically rigid and flexible substrates. 44 SEM image of the above-mentioned GaS NS, illustration of the device structure

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photocurrent of 0.115 mA which is 107 times higher than that of graphene-free UV

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and photocurrent as function of time are given in Figure 5 (a-c). The photoresponsivity of GaS NS on SiO2/Si substrates and on polyethylene terephthalate at 254 nm yielded up to 4.2 higher than that of pristine graphene photodetector (1x10-3 AW-1 with external quantum yield of 6-16%).

4, 44

It was evident from theoretical modelling that the decrease in sheet thickness

of GaS NS caused by the reduction of effective mass at valence band maximum increases the carrier mobility leading to high photocurrent.

44

Yang et al.

45

reported few-layer GaS two-

terminal photodetector with a fast and stable response showing different photo-responses in diverse gas environments (Refer Figure 5 (d-f)). The photo responsivity and external quantum efficiency is found to be higher in ammonia rather than in air or oxygen. The charge transfer between the adsorbed gas molecules and the photodetector resulted in different photo-responses. The photo-response of the above-mentioned GaS NS based device (64.43 AW-1) 45 was found to be higher than the pure monolayer graphene photodetector (8.61 AW1 76

)

and graphene nanoribbons (1 AW-1). 134

Gallium selenide (GaSe) is a layered material belongs to the TMDCs family which comprised of vertically stacked Se-Ga-Ga-Se sheets held together by weak forces such as van der Waals forces along with some ionic or Coulomb contributions.

135

This p-type semiconductor

material has the indirect bandgap of 2.11 eV which is very close to the direct bandgap. 136 In this case, thermal vibration at room temperature is enough for the transfer of electrons between between direct and indirect band edge because of small difference (25 MeV). Singleand few-layer GaSe 2D ultrathin nanosheets obtained from mechanical cleavage and solvent exfoliation method were employed for fabricating photodetector. 79 It showed a fast response of 0.02 s, high responsivity of 2.8 AW-1 and high external quantum efficiency of 1367% at 254 nm. As the wavelength of light decreased from 610 to 254 nm, a significant increase in photocurrent was observed particularly when the device is illuminated with short-wavelength light. Figure 5 (g-i) depicts the atomic force microscopy (AFM) image of few-layer GaSe flake on a silicon substrate with a 300 nm thick oxide layer, schematic of GaSe photodetectors and photocurrent as function of time at bias voltage of 1 V, respectively. Zinc selenide (ZnSe) being II–VI compound semiconductor having direct bandgap of ~2.70 eV (~460 nm) at room temperature (RT) is widely adopted material in the realization of optoelectronic devices. 50, 137 It is more sensitive to blue/UV light compared to that of silicon and gallium arsenide. 1D ZnSe nanostructures were limited by their high dark currents or poor stabilities which impose challenge in fabricating efficient photodetector with high-

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AW-1 and 19.2 AW-1 (with external quantum yields up to 9374%), respectively which is

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performance characteristics of reproducible photocurrent, high photocurrent/dark-current ratio and fast response time.

138

In this scenario, Fang et al.

50

reported ultra-long, easily

the fabrication of blue/UV-light-sensitive photodetectors (λ<460 nm) with ultralow dark current, a high photocurrent immediate decay ratio (>99%) and fast time response (<0.3 s). The decrease in dark current is contributed to the perfect crystallinity of the chemically pure ZnSe structures. The typical SEM image of ZnSe nanostructures showing belt-like morphology with nanobelt thickness as ~ 40 nm, schematic illustration of the single ZnSe nanobelt fabricated as a photodetector and the I–V characteristics of the device illuminated with 400-nm wavelength light and under dark conditions can be well understood from Figure 5 (j-l). Niobium pentoxide (Nb2O5) belongs to transition metal oxides with eminent applications in different

fields

such

as

field-emission

displays,

catalysis,

electrochemistry

and

microelectronic devices. It can also be employed in the fabrication of optoelectronic circuit based devices (UV-A band of 320-400 nm) and visible blind VU light sensors ascribed to its bandgap of 3.4 eV.

139

Fang et al.

52

reported a nanoscale photodetector fabricated from an

individual Nb2O5 nanobelt of 100-500 nm wide and 2-10 µm long. The Cross-sectional SEM image of Nb2O5 NB arrays, schematic diagram of single Nb2O5-nanobelt device and the photocurrent-time plot can be seen in Figure 6 (d-f). The detector showed high UV-Alight sensitivity, high external quantum-efficiency and excellent photocurrent stability of more than 2500 s. These UV photodetectors have excellent optoelectronic properties confirmed their applicability as visible-blind UV-light sensors and optoelectronic circuits particularly operating in the UV-A range. UV photodetectors fabricated on single Nb2O5 nanoplate showed excellent sensitivity, high external quantum efficiency, robust stability, steady photocurrent dependence on light intensity and wavelength selectivity with respect to UV-A light.

140

Figure 6 (a-c) details the SEM image, pictorial representation of Nb2O5 nanoplate

photodetector and time-dependent responses of the photodetector upon illumination of the 320 nm light measured using the mechanical chopping method, respectively. UV photodetectors based on WO3, HfS3, InSe and In2Ge2O7 are also joining the race of photo-detection. WO3 belongs to TMDCs transition metal oxides having bandgap of ~3.3 eV with high chemical and thermal stability most suitable UV photodetector application. Few-layer WO3 nanosheets based UV photodetectors

144

141-143

showed high-performance with a

faster photoresponse and higher EQE as compared to that of previously reported WO3

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tuned/manipulated ZnSe nanobelts synthesized by en-assisted ternary solution technique for

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nanowires

142, 143

and nanospheres.

141

Recently, HfS3 nanobelts of 700-70 nm width, 10-30

nm thick and 10 µm in length with direct and indirect optical energy gaps of 2.19 eV and 1.73 145

These devices have proven with

exceptional photoresponse from ultraviolet to visible light. The solar-blind photodetectors based on individual single-crystalline In2Ge2O7 nanobelts as sensing elements exhibited sensitive spectral response and high quantum efficiency.

146

Figure 6 (g-i) shows the SEM

image of In2Ge2O7 nanobelts, schematic of the NB device and the time response of an In2Ge2O7-nanobelt photodetector, respectively. The orthorhombic layered structure together with layer-dependent direct bandgap from monolayer to bulk favors BP for photodetection. More recently, Wu et al.

147

reported the optoelectronics characteristics of high-quality, few-

layer BP-based colossal UV photodetectors. Scanning measurement using AFM across the channel area of device showed the thickness of BP flake as ∼4.5 nm as seen in Figure 6 (j). The 3D view of BP photo-FET device structure to measure photoresponse and the photoresponsivity of BP device for the combination of fast and slow response of photocurrent with excitation of 390 nm light source at the same applied back gate is given in Figure 6 (k,l). UV photoresponsivity (∼9 x 104 AW-1) was achieved and it is the highest ever measured in any 2D materials till now with 107 times more compared to previous research on BP. 86, 90, 98, 99, 148

The resulted colossal UV photoresponsivity was ascribed to the resonant-interband

transition between two specially nested valence and conduction bands. They have concluded that these nested bands yield unusually high density of states for highly efficient UV absorption because of their singularity nature. 3.3 Visible Photodetectors Cadmium sulphide (CdS) is a considered to be promising material for visible photodetectors with bandgap of ~ 2.4 eV.

23, 46, 149, 150

It is challenging to fabricate a high-performance

photodetectors with fast response time, good reproducibility and high quantum efficiency by overcoming low quantum efficiencies and slow response speeds so that the practical demands could be met.

46, 149, 151, 152

Li et al.

23

reported ohmic contact-based single CdS NB

photodetectors with ultrahigh responsivity and fast response time. The high-resolution SEM image of CdS NB represents that the belt-like structure have flat surface with uniform belt thickness of ∼200 nm as seen in Figure 7 (a). Figure 7 (b, c) reports the schematics of CdSNB photodetector and reproducible on/off switching (rise and decay times were ∼ 20 µs) upon 490 nm light illumination. Jie et al.

46

reported the photoconductive characteristics of

CdS NRs whose SEM morphology is given in Figure 7 (g). From the Figure 7 (e), we can

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eV, respectively is devised as flexible photodetector.

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better understand the I-V curves illuminated with light of different wavelength where the insets show the optical microscopic image of the single-NR device (left) and the schematic CdS NR to the pulsed incidence light (490 nm) power density of 13.4 kW/cm2 is depicted in Figure 7 (f). The systematic study of spectral response, light intensity response and time response in single CdS NRs reported a fast response speed where the size of NRs played a significant role. In addition, the smaller sized (width and thickness) CdS NRs showed higher response speed. Furthermore, large surface-to-volume ratio, high single-crystal quality of CdS NRs and the reduction of recombination barrier are responsible for the high photosensitivity and high photoresponse speed. conductance with a response time of 1-3 s.

149

46

CdS NBs have a noticeable increase in the

Hence, it has the potential of realizing nano-

photoconductors and optical switches by incorporating these NBs. The high sensitivity and quick response in the visible range of single CdS NBs were contributed by fast recombination and trapping in which the surface traps also influenced the photoresponse. 153 It was revealed that the high photoconductance, the short rise time and decay time of CdS NRs are significant for their use in photo-detection. 152 The single CdS NB metal-semiconductor field-effect transistors (MESFETs) based photodetectors was reported with the novel advantage of an additional dimensionality in controlling the channel conduction.

154

Figure 7 (d-f) shows the field emission SEM

(FESEM) image, schematic illustration of individual CdS NB MESFET based photodetector and ON/OFF photocurrent response of CdS NB without Schottky contact as a function of time. In view of this, the dark current is greatly reduced thereby increasing the photoresponse so that it can overcome the disadvantage of conventional two-terminal photodetector. Hence, the overall performance of the photodetector can be enhanced by the realization of MESFET based CdS NB photodetector. Rare earth (RE) doping can serve as effective luminescent centers attributed to the recombination of the photogenerated carriers confined in semiconductors and subsequent energy transfer to RE ions leading to the excitation of charge carriers in RE ions and this finds application in multicolor photodetectors. 156

155

Dedong et al.

3+

reported Er -doped CdS nanoribbons (Er:CdS NRs) based photodetector with the ability

of detecting multicolor light of blue, red, and near-infrared light with higher responsivity and external quantum efficiency. Figure 8 (a-c) portrays the SEM image of single Er:CdS NR at high magnification, schematic of the detector configuration and I-V curves of Er:CdS NR device under illumination with an incandascent lamp. The superior performance of the Er-

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diagram of the configuration for photoconductive measurements (right). The time response of

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CdS NR device offers an avenue to develop highly sensitive multicolor photodetector applications. Self-powered photodetectors based on gallium doped cadmium sulphide 157

FESEM image of Ga:CdS

NRs, energy band diagrams of open-circuited and short-circuited Ga:CdS NR/Au Schottky junction device under light illumination and the time response (95/290 µs) at zero bias under light illumination of 510 nm are illustrated in Figure 8 (d-f).Moreover, the photoresponse of Schottky junction photodetectors at zero bias had excellent repeatability and stability with fast response speed in a wide range of switching frequencies. Cadmium selenide (CdSe) being a direct bandgap II-VI semiconductor with bandgap of ~1.797 eV (wurtzite) and ~1.712 eV (zinc blende) covers a broad range of visible spectrum. 158

CdSe nanobelts and nanosheets are promising towards the application of optical and

sensory devices. 159 The potential of CdSe nanoribbons in high-sensitivity photodetectors and photoelectronic switches was reported. Photodetectors made from CdSe nanoribbons showed high sensitivity, high photo-to-dark current, high response speed, excellent stability and reproducibility. The presence of discrete traps at different energy levels in the bandgap is caused by the light-dependent response speed.

61

The impurity dependent optoelectronic

properties resulted from the impurity traps in CdSe NB caused different photoresponse behaviour. The intrinsic CdSe NB is found to be more suitable for fast and sensitive photodetector applications in contrary to n-type CdSe NB that is favourable for high-gain photodetector applications.

60

Figure 8 (j-l) depicts the FESEM image of CdSe NBs,

schematic diagram of experimental setup adopted for studying the photoresponse speed of CdSe NB PDs and the reproducible light ON/OFF switching properties of the two devices as reported by Peicai et al.

60

Moreover, ohmic and Schottky contact based CdSe NB are used

for photodetector fabrication in which the sizes of i- and n-NBs are almost identical so that the size-dependent effect can be minimized. High performance CdSe NB MESFET based photodetectors with high current responsivity, high gain and and fast photoresponse speed was also reported.

160

Plasmonic photodetector (PPD) based on sole CdSe NRs and CdSe

NRs decorated by plasmonic hollow gold nanoparticles (HGNs) on the surface with high sensitivity to red light illumination that can couple the incident light due to localized surface plasmon resonance (LSPR) was demonstrated.

161

HGNs can function as an “antenna” for

improving photocurrents owing to the plasmonic nearfield coupling. It also possibly enhances light absorption by localizing the incident field. The light illumination of as-formed Schottky barrier leads to the energetic hot electrons from LSPR excitation of metallic plasmonic

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(Ga:CdS) NRs/Au Schottky barrier diodes were investigated.

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nanoparticles to couple with the resonant photon energy which further transfers to the nearby CdSe NR. TEM image of individual CdSe NR, schematic illustration of PPD and

are detailed in Figure (g-i). The significant improvement of responsivity, ON/OFF current ratio and detectivity occurred via altering the HGNs. The optimized performance of the device was attributed to localized field enhancement and direct electron transfer induced by HGNs. Gallium telluride (GaTe) exist in III–VI layered semiconductors having direct bandgap (~ 1.7 eV at RT in both single layer and bulk) with monoclinic crystal structure in the space group C2h3 and individual layers were bonded to each other by weak van der Waals’ interactions. The Ga-Ga bonds in GaTe belong to two categories: one-third are parallel and two-thirds are perpendicular to the layer. GaTe achieved great interest in the demonstration of optoelectronic applications because of their direct band structure either in bulk or in single layer. GaTe nanosheets on mica of few tens of µm to 100 µm were adopted for photodetector. 162

Figure 9 (g-i) depicts the SEM image of GaTe NSs on SiO2/Si substrate grown by CVD

method, optical images of the GaTe nanosheet device on a polyethylene terephthalate substrate and time-dependent photoresponse of the same device with the laser on and off after bending different times at a bias of 5 V. It was found that the adsorbates on the GaTe surface influenced the performance of the device and a fast response GaTe photodetectors was achieved after the removal of adsorbates in ∼7×10−5 torr vacuum. Hu et al. 163 reported GaTe nanosheet based phototransistors in which the performance (photoresponsivity, detectivity) can be tuned by the applied gate voltage. Figure 9 (a-c) shows the SEM image of GaTe NS, schematic of PD (Inset: GaTe NS device) and also the time-resolved photoresponse at different bias voltage Vds = 0.1, 0.5 and 1 V. The detectivity of these devices was found to be ~ 1012 Jones, which exceeded the InGaAs photodetectors (1011 ~ 1012 Jones).

164

The direct

bandgap structure of GaTe nanosheets played a major role in increased photon absorption and generation of excitons. The reported GaTe nanosheet photodetectors showed a responsivity of 274.4 AW-1 that surpassed the previously reported 2D nanosheet devices based on graphene, GaSe and GaS.

44, 75, 79

Liu et al.

165

studied high-sensitivity photodetectors based on

multilayer GaTe flakes with high photoresponsivity (104 A/W) and the fast response time (6 ms). The optical image of multilayer GaTe device, its 3D schematic view and time-resolved photoresponse are shown in Figure 9 (d-f). The reported photoresponsivity was higher than that of graphene, MoS2, and few layered compounds. 111

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photoresponse of pure CdSe NR and HGNs@CdSe NR PD under 650 nm light illumination

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The direct bandgap (1.44-1.50 eV) of cadmium telluride (CdTe) along with their interesting photoresponse characteristics make them suitable for photodetection in visible-near infrared 51

reported photodetectors based on single CdTe NRs

with high responsivity, gain, high stability and reliability. The influence of surface states of NRs, channel length, light intensity, and working bias voltage on the photoresponse characteristics was also analysed. It has been demonstrated that surface state-induced traps is a significant factor to attain high responsivity and gain. The channel length was reduced with the increase in bias voltage which was responsible for responsivity and gain enhancement whereas the response speed was reduced due to the high trap density. Zirconium trisulphide (ZrS3) belongs to transition metal trichalcogenides having a direct optical bandgap of 2.56 eV at room temperature. Flexible visible-light photodetectors based on ZrS3 nanobelt films were fabricated with broad photoresponse, excellent photo-switching effect and stability.

16

The as-fabricated devices showed a tunable spectral selectivity, wide-

range photoresponse, high-speed response and excellent environmental stability. With the illumination of light, currents are higher at the similar bias voltage in the wavelength range of 405 nm to 780 nm with respect to that of dark state which explains the photosensitivity of ZrS3 nanobelt based device from the visible light to the near infrared. The photocurrent induced by 405-nm light at 45 mW cm2 is greater than the 780-nm light at 49.9 mW cm2 at the same voltage.

16

This is an evident for dependence of photosensitivity on the optical

intensity and wavelength of the light. The response of the device in air and vacuum was studied in order to understand the photosensitive mechanism. The current in vacuum is found to be higher than that in air which shows the presence of oxygen chemisorption/desorption on the ZrS3 nanobelts. The oxygen molecules are adsorbed on the surface of nanobelt in the presence of air so that they can attract free electrons from NB (form O2-) to form a lowconductivity depletion layer adjacent to the surface. The electron-hole pairs are generated in the device under illumination of light. The migration of holes towards the surface recombines with O2- form O2 that desorbs from surface. This causes an increase in concentration of electron thereby increasing the photocurrent. The response speed was found to be faster than in air. Hence, the nanobelt-film-based photodetectors show enhanced stability and durability compared to a single nanowire photodetector. Therefore, they could be attractive candidates for high-performance nanoscale optoelectronic devices for photodetection of visible to near infrared light. Xiong et al.

166

reported photodetectors based on individual nanobelts of

zirconium triselenide (ZrSe3) and halfnium triselenide (HfSe3) NB photodetectors which

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(NIR) region (400-800 nm). Xie et al.

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showed a good photoresponse to wavelengths ranging from 405 nm to 780 nm, high sensitivity, stability and a fast response to visible light.

of ZrS2 nanobelt FET device and their reproducible on/off switching upon 450-nm light illumination are given in Figure (g-i). Photoconductivity of individual ZrS2 nanobelts showed excellent visible-light sensing performance.

167

Monolayer MoS2 phototransistors with

improved device mobility and ON current showed a maximum external photoresponsivity of 880 AW-1 at wavelength of 561 nm and photoresponse in 400–680 nm range was demonstrated.

168

Figure 10 (a-c) presents the optical image of single-layer MoS2 flake and

device, schematic view of the photodetector (c) Cross-sectional view of the PD structure together with electrical connections. MoS2 nanosheet based phototransistor with prompt photoswitching and good photoresponsivity was also investigated.

169

AFM image of single-

layer MoS2, optical image of FET device made by single-layer MoS2 and photoswitching characteristics of the same device at different optical power and drain voltage are depicted in Figure 10 (d-f). Molybdenum doped rhenium selenide (Mo:ReSe2) is optically biaxial and has relatively high crystal symmetry (CdCl2-type octahedral structure of triclinic symmetry) that makes it differ from most of the other hexagonal layered TMDCs.

170

It is an octahedral structured

semiconductor that has optically biaxial and highly anisotropic nature. As the crystals are optically biaxial, a clustering of Re4 diamond units form along the b-axis within the van der Waals plane in Mo:ReSe2 monolayer which is in contradiction with hexagonal structured TMDCs with their optical axis perpendicular to van der Waals plane i.e. optically uniaxial. 171, 172

This phenomenon of “diamond chains” clustering accounts for their in-plane optical

and electrical anisotropic response and is the reason for their usage in the fabrication of polarization sensitive optoelectronic devices.

photodetectors, 170, 172

photo-electrochemical solar cells and

other

The investigations on the effects of physisorption of gas

molecule on the few-layer Mo:ReSe2 nanosheet based photodetectors was made by comparing the photoresponse of the as-exfoliated device with annealed device both in air or ammonia. 170 AFM image of these few-layer Mo:ReSe2 NS along with their device schematic and examination of the On/Off responsivity are given in Figure 11 (d-f). These photodetectors after annealing in ammonia atmosphere displayed enhanced photoresponsivity and EQE than that of air and is evident that charge transfer between NH3 molecule is caused by physisorption of NH3 molecule thereby increasing the n-type carrier density.

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The high-magnification SEM image of zirconium disulphide (ZrS2) NB, schematic diagram

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Engel et al.

173

reported multi-layer BP photodetector to record diffraction-limited images of

microscopic patterns in the visible (λVIS=532 nm) and infrared (λIR=1550 nm) spectra. with model point spread functions. The experimental resolution (270±5 nm) comply well with the width of the model point spread function at λVIS in which estimate of the theoretical resolution limit is ~260 nm. Similarly, obtained experimental resolution (720±15 nm) agrees well with the width of model point spread function at λIR in which the theoretical estimate is ~760 nm. Hence, BP photodetector can be employed for the acquisition of diffraction-limited microscope images in both visible and infrared spectral domain. AFM image of the abovementioned BP PD and schematic of the imaging process (500 nm square array) are shown in Figure 11 (g,h). Figure 11 (i) depicts the scanning photocurrent micrographs of the shortcircuit photocurrent excited at λVIS=532 nm with an optical power density of 13.4 kW/cm2. Photodetectors based on single-crystalline germanium monosulphide (GeS) nanoribbons having a thickness of 20–50 nm, a width (several µm) and a length of hundreds of micrometers responded well to the entire visible light with a response edge at around 750 nm reliable with the bandgap of GeS.

174

Figure 11 (a-c) depicts the SEM image of GeS

nanoribbons, the fabricated photodetector and their time dependence with a bias voltage of 5 V under 530 nm light illumination. GeS is a layered p-type semiconductor with distorted rock-salt orthorhombic structure and bandgap of 1.65 eV.

175, 176

Graphene nanoribbon

decorated with MoS2 as carrier transport channel and MoS2 nanoparticles were employed to develop flexible (bending radii > 6 mm) and low-cost photo-transistors exhibited photoresponsivity of 66 A/W.

53

Graphene nanoribbon acted as carrier transport channel and

MoS2 nanoparticles supplemented high gain absorption thereby generating the electron-hole pairs. 3.4 IR and Terahertz Photodetectors Germanium selenide (GeSe) is a IV−VI p-type narrow band gap (1.08 eV) semiconductor with layered crystal structure and has a high anisotropic crystallization in layered structure with layers in parallel to growth direction.

177

Schottky photodetector based on single-crystal

GeSe nanosheets combined with Au contacts showed NIR light illumination (maximum photoconductive gain ∼5.3 × 102 % at 4 V) at a wavelength of 808 nm.

178

Figure 12 (a, b)

represents the low-magnification SEM images of GeSe nanosheets and performance of GeSe nanosheet-based photodetector device under 808 nm-light illuminations, respectively. The time-dependent photocurrent response of the device to laser light illumination with different

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Diffraction-limited images were confirmed by correlating the measured edge cross sections

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light intensities under vacuum (4 × 10−3 mbar) at fixed bias of 4 V is given in Figure 12 (c). The defects-induced in-gap defective states caused slow decay of current in the OFF state and

calculations. Photodetectors integrated with five MoS2 monolayers exhibit a high photoresponsivity of 1.8 A/W, an external quantum efficiency > 260%, and photodetectivity of ~5 x 108 Jones for wavelength of 850 nm.

179

Youngblood et al.

148

reported gated

multilayer black phosphorus photodetector integrated on a silicon photonic waveguide operating in the NIR telecom band. Figure 1 (d) shows the three dimensional (3D) representation of the waveguide integrated BP device configuration with a few-layer graphene top-gate. The optical microscopy image of the final device is given in Figure 1 (e) where the BP field-effect transistor is incorporated in one arm of Mach-Zehnder interferometer circuit. Armchair graphene nanoribbons (AG-NRs) is found to be a great substitute in infrared photodetectors owing to their tunability of bandgap by changing the width, number of layers and inducing a transverse electric field on the structure. The photodetector parameters such as quantum efficiency and optical responsivity of AG-NRs p–i–n structure with all three structural families, different width and different number of layers to use in IR detectors was studied tight-binding model.

180

The responsivity of AG-NRs based IR photodetector was

found to increase by increasing the width and number of layers and it was decreased by increasing the temperature. 180 Theoretical studies on dark current of IR photodetectors based on AG-NRs with its dependence on gate voltage, width of nanoribbon and temperature was demonstrated. 181 Moreover, carriers to the AG-NR area were induced by the gate voltage in which photo-excited carriers sweep out of the detection area to produce an output signal. The results revealed that the dark current was increased with the increase in temperature. The width of the AG-NRs also influenced the dark current and it can be explained as follows: (1) In narrow AG-NRs, the dark current was increased by increasing the gate voltage and (2) In wider AG-NRs, the dark current was decreased by increasing the gate voltage.

181

Optical

properties of armchair graphene nanoribbons embedded in hexagonal boron nitride (BN) lattices studied by tight binding calculations showed that larger photo current could be achieved in AG-NR/BN structures compared to conventional GNRs attributed to the more allowed transitions. 182 Ryzhii et al.

183

reported the new concepts of terahertz and infrared photodetectors based on

multiple graphene layer and multiple graphene nanoribbon structures and concluded that their

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weak light intensity dependence of photocurrent and was confirmed from the first-principles

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responsivity and detectivity was caused by the high values of quantum efficiency and low rates of thermogeneration. Monolayer graphene has high quantum efficiency of interband

in the number of graphene layers, the responsivity of multiple graphene layer photodiodes increases sub−linearly thereby increasing the quantum efficiency. The responsivity is very high in case of sufficiently large number of graphene layers owing to strong interband absorption. Moreover, the detectivity being a function of number of graphene layers attain maximum and slightly decreases afterwards.

183

The mechanisms of photoconductivity in

graphene layer-graphene nanoribbon-graphene layer (GL-GNR-GL) structures having i-type gapless GL as sensitive elements and I-type GNRs as barrier elements by developing a device model for GL–GNR–GL photodiode was demonstrated.

186

The dark current, photocurrent,

and responsivity are calculated as functions of the structure parameters (width of GNR, doping causing change in lattice constant), temperature, and the photon energy by means of the above-mentioned model. Their studies suggested that GL–GNR–GL photodiodes could be an interesting candidate for the fabrication of infrared and terahertz detectors operating at room temperature. The theoretical concept of terahertz and infrared photodetectors using the resonant radiative transitions between graphene layers in double-GL structures was also proposed by Ryzhii et al.

187

The calculated absorption spectrum and the spectral

characteristics of the reported photodetector responsivity showed sharp resonant maxima (tunable by applied voltage) at the photon energies in a wide range. The working of double graphene layer PDs is correlated to the absorption of incident IR radiation together with the electron tunneling transitions between GLs for producing electric terminal current. Chitara et al. 134 established IR photodetection using both reduced graphene oxide (RGO) and GNRs in terms of time-resolved photocurrent and photoresponse. The TEM image of GNRs, highresolution (HRTEM) image (inset) along with photodetector mechanism and photocurrent as a function of time with different IR intensities at 2 V for GNRs are presented in Figure 12 (df). The responsivity and external quantum efficiency of RGO were 4 mA/W and 0.3%, respectively, whereas for GNR these values were 1 A/W and 80%, respectively, for an incident wavelength of 1550 nm at 2 V. Graphene has optical absorbance of 2.3% +/- 0.2% and the value is doubled for bilayer graphene.

188-192

Recent report showed that the optical

absorbance spectrum of few-layer graphene depends on the stacking sequence of monolayer graphene. 193 Hence, GNRs could be a better material for high-selectivity, high sensitivity and high-speed nanometer-scale photodetectors and photoelectronic switches.

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transitions compared to bilayer graphene and graphene nanoribbons. 184, 185 With the increase

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Recently, researchers are more oriented towards graphene plasmonics and it is an interesting research field which is evident from the publications.

194-204

Interestingly, van der Waals 205

Graphene

plasmons can be excited by electromagnetic radiation if and only if their wave vector, energy and optical fields are matched. This can be performed in different methods: (i) enhanced plasmonic extinction by light scattering in the vicinity of the sub-wavelength nano-scale structures,

206

(ii) coupling by grating developed on either top or bottom of graphene

by patterning the graphene to form periodic plasmonic metamaterials.

208-210

207

(iii)

Graphene’s

optical properties in the infrared and terahertz could be altered/improved by sculpting graphene into periodic metamaterials with sub-wavelength feature sizes. Moreover, arrays of graphene nanoribbons, nanodiscs or nanorings have the ability to support localized plasmonics thereby acting as sharp notch filters in the mid-infrared. Freitag et al.

211

realized

polarization-sensitive and gate-tunable plasmonic photodetection in graphene nanoribbon arrays in which the photo-induced temperature increases up to four times as large as comparable two-dimensional graphene detectors. The SEM image of close-up of the contact area of a GNR array photodetector having 100 nm GNR width and 100 nm GNR spacing are shown in Figure 12 (g). The length and width of the whole array was given as 30 and 10 mm, respectively. Figure 12 (h) shows the schematic diagram of the photoconductivity setup in which the IR laser light at 10.6 mm was chopped at 1.1 KHz and the photocurrent was analysed by a lock-in amplifier referenced to the chopping frequency. The scanning photocurrent image of GNR superlattice PD is depicted in Figure 12 (i). Here, novel roomtemperature mid-infrared detector based on the intrinsic graphene plasmon excitations was demonstrated by altering the graphene plasmons by means of remote plasmon–phonon coupling with the surface phonons of the polar substrate. 4. Outlook and Challenges The next significant technological step might be the nano-optoelectronic integration of hybrid devices in realization of photodetectors. The size of NB/NR/NS tunes their bandgap that can be the key factor for spectral tunability within the single material system in the process of multispectral photodetectors that are sensitive for visible, near-infrared, ultraviolet and broadband spectral range. One can tune the bandgap if the structures are formed in core-shell form and also there are more opportunities for this kind of research as it combines two different material. The potential of doping strategies in controllable manner during the time of growth to achieve uniform distribution in NB/NR/NS could also be considered.

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heterostructures are also growing in the field of mid-infrared nanophotonics.

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NB/NR/NS are rewarded with bounteous enhanced properties of quantum confinement effect, dimensionality dependence, compact size, and ease of manipulation, precise crystal

exposure to light, strong polarization dependence, better photoresponse and the response speed. The design of ternary compounds in these materials will be of great challenge in order to understand the tuning of photo-detection range by taking into account of the solid solution formation. The fabrication of complex hybrid nanostructured materials based photodetectors are blooming nowadays. The sustained progress in this field and the future for these devices will demand new paradigms. It is more challenging to form precise hybrid nanostructures with more focus of feature size, dimensionality and distribution of materials will pave the way for attaining better performances in photo-detection. The two-dimensional nanostructures can be more compatible with the current thin film micro-manufacturing techniques along with their easy fabrication into complex structures owing to their dimensionality, compact size, ease of manipulation, precise crystal structures, rationally designed surface and peculiar onedimensional enclosing surfaces. In this way, one can overcome the limitations such as reducing the dimensions to nanoscale, controlling the crystallinity, surface to volume ratio, lattice mismatch, no well-defined interface and purity. The novel hybrid nanostructures will be a great boom for the realization of novel photodetectors with enhanced properties to achieve better performances. The new innovations should be more focussed on grasping these plentiful properties to upgrade the device fabricating techniques in terms of bandgap engineering. There is still plenty of room for the design of smart, flexible, self-powered, portable, multifunctional photodetectors in the emerging area of nano-optoelectronics. More future works are encouraged in this specific theme dedicated towards the real-time applications to be available for the common man in the society. Acknowledgements This work is partially supported by the National Natural Science Fund of China (Grant No. 61222505 and 61435010), 863 Program (Grant No. 2013AA031903), the youth 973 program (Grant No. 2015CB932700), Natural Science Foundation of Guangdong Province of China (Grant No. 2014A030310416)

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structures, rationally designed surface, peculiar 1D enclosing surfaces, large surface area

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Table 1: Important parameters of nanostructured photodetectors Photodetector Material Type

Responsivity (A/W)

EQE (%)

Response time

Broadband

~ 56

~ 10200

0.4 x 10-3 4.8 x 10-3 0.418, 3.54

-

2570 5.68 x 104

-

3.574 x 104

-

2.975 x 103

-

2.06 x 10-4 4.7 x 10-7

MoO3 nanosheets/nano belts Layered BP Few-layer BP BP/monolayer MoS2 Few-layer MoS2 InSe NS (λ=254) InSe NS (λ=490) InSe NS (λ=850) SnS2 NS (λ=365) SnS2 NS

Ref.

-

ON/OF F current ratio -

1 ms

> 103

98

32

86 90

0.3 5 ms, 8 ms ~ 107

114

-

-

119

-

-

2.63

115

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Ultra-Violet

Visible

(λ=405) SnS2 NS (λ=532) SnS2 NS (λ=650) SnS2 NS (λ=780) SnS2 NS (λ=850) G-Bi2Te3 GaS nanosheet GaS nanosheet GaSe nanosheet ZnS/ZnO NB ZnS NBs ZnSe NB Few-layer BP Sb:ZnO NB WO3 nanosheet In2Ge2O7 HfS3 NB Nb2O5 NBs Nb2O5 nanoplate ZrSe3 nanobelts HfSe3 nanobelts ZrS2 nanobelts CdS nanobelt CdS nanoribbon CdS nanobelt CdS nanoribbon CdS nanobelt Ga:CdS NR

Er:CdS NR (λ=457.5 nm) Er:CdS NR (λ=620 nm) Er:CdS NR (λ=955 nm) CdSe nanobelt CdSe nanoribbon CdSe nanobelts HGNs@CdSe NR

1.13 x 10-6

-

-

6.12 x 10-6

-

20/31 s

2.68 x 10-6

-

-

1.22 x 10-8

-

35 5.06 x 10-4 64.43 2.8 5 x 105 ~ 0.12 0.12 ~ 9 x 104

9374 12621 1367 2 x 108 37.2 -

293 3.9 x 105 ~ 6 x 10-5 15.2 24.7

997 2 x 108 6070 9617

26.5/26.7 s < 0.03 s ~ 10 ms 0.02 s < 0.3 s <0.3 s ~ 1 ms, 4 ms < 100 ms 80 ms ~ 3 ms 0.2 s

0.53 0.012 7.1 x 105 ~ 7.3 x 104 ~ 2 x 102 ~ 4, 6, 8 (Ga doping at 2%, 4%, 8%) 3.46 x 104

~8 -

84

133

28 s, 12 s

12 -

101 2.8 1.8 x 108 1.9 x 107 104 ~ 5.2 x 102 -

< 0.4 s < 0.4 s ~ 2 µs ~ 20 µs >500 µs >1 s >700 µs -

~ 1.3 103

166

93800

-

103

156

8.14 x 103

16280

-

102

156

9.19 x 103

11930

-

10

156

~ 1.4 x 103

-

5 x 108

160

-

~ 2.7 x 103(Gain) -

>1 ms

-

61

4.87 x 104

-

~ 15 µs -

-

60

44 45 79 36 49 50 147

144 146 145 52 140

166 167 23 46 149 152 154 157

161

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CdTe nanoribbon Mo:ReSe2 NS Single layer MoS2 GaTe NS GaTe NS

IR

GaTe Flakes Graphene NR@MoS2 Few-layer GeSe MoS2 BP Graphene NR

7.8x102

3700

~ 55.5 4.2 x 10-4

10893 -

0.03 274.4

8 -

104 66

-

3.5 1.8 135, 657 1

537 260 10, 50 80

~ 1.1 s / ~ 3.3 s < 100 ms < 0.05 s

48 ms, 150 ms 6 ms 5 ms, 30 ms 100 ms 0.3 s -

-

51

20 -

170

11 -

162

-

165

500 -

178

169

163

53

179 148 134

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Figure 1. State-of-the-art Photodetector devices using BP- (I) BP-MoS2 heterostructure: (a-c) Schematics and optical image of the fabricated device structure where dark purple region corresponds to monolayer MoS2, blue flake represents few-layer BP and light purple region is SiO2 (scale bar 10 µm), Reproduced from Ref 90 (II) Waveguide integrated BP-G: (d) Representation of the device showing a few-layer graphene top-gate, (e) Optical microscope image of final device. (d,e) Reproduced from Ref 148

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Figure 2. Broadband photodetectors based on BP- (I) BP photodetector: (a) Optical image with Ring-shape photocurrent collector, (b) Polarization dependence of the photoresponsivity and (c) Gate enhanced photoresponsivity for linear dichroism detection, (ac) Reproduced from Ref 98, 99 (II) Black-phosphorus (b-P) based FET: (d) Optical image, (e) Schematics and circuit used to perform the two terminal electrical measurements and (f) Photocurrent as a function of time under modulated light excitation (~20 Hz) with different wavelengths. The shaded areas indicate the time when the light beam is blanketed by the mechanical chopper. (d-f) Reproduced from Ref 86 (III) BP/MoS2 p-n heterojunction photodetector: (g) Optical image (scale bar 5 µm), (h) Photocurrent mapping under zero volt bias where the dashed lines outline the heterojunction regions, solid lines outline the metal contacts A and B that are electrically connected to BP and MoS2, respectively and (i) Photodetection responsivity as a function of incident power. (g-i) Reproduced from Ref 90

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Figure 3. Broadband photodetectors based on (I) MoO3 nanobelt: (a) SEM image, (b) Schematic illustration of device for photocurrent measurement and (c) Time dependent photoresponse before and after annealing under 660 nm laser illumination. (a-c) Reproduced from Ref 32 (II) Triple-layer MoS2: (d) Optical image of the whole device having MoS2 with Cr/Au contact (Scale bar, 10 µm), (e) Schematic view of photodetector with monochromatic light beam and (f) Photocurrent response. (d-f) Reproduced from Ref 114 (III) Bi2Te3 nanoplatelets on graphene: (g) SEM image, (h) Schematic of the heterostructure photodetector device and (i) Photocurrent as a function of gate voltage. Inset: Energy diagrams of the heterostructure where VgVd. (g-i) Reproduced from Ref 84 (IV) InSe nanosheets: (j) Schematic drawing of a phototransistor. Inset: Typical image, (k) Schematic of the photon absorption of InSe multilayer and (l) Band diagram of InSe nanosheet phototransistor: Ef is the Fermi level energy, Ec the minimum conduction band energy, Ev the maximum valence band energy and (ɸB) the barrier height. (j-l) Reproduced from Ref 115

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Figure 4. ZnS nanobelts based UV Photodetectors- (I) ZnS nanobelts: (a) TEM image of ZnS nanobelts (Inset: SEM image of a single-crystalline ZnS-nanobelt device), (b) Schematic diagram of device and (c) Time response upon 320 nm light illumination with and without UV light. (a-c) Reproduced from Ref 49 (II) S-G/ZnS nanobelts: (d) TEM image, (e) Schematic diagram of device and (f) Energy level diagram of S-G/ZnS heterojunctions presenting charge-transfer process under UV-light illumination. (d-f) Reproduced from Ref 127 (III) ZnS/ZnO biaxial nanobelts: (g) SEM image of ZnS/ZnO nanobelts (Inset: Highresolution SEM image), (h) SEM image of the device and (i) Schematic energy band diagram after UV irradiation. (g-i) Reproduced from Ref 132

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Figure 5. Nanostructured UV Photodetectors- (I) GaS nanosheet: (a) SEM image of GaS NS (b) Illustration of the device structure (Inset: optical image of device) and (c) Photocurrent as function of time. (a-c) Reproduced from Ref 44 (II) GaS nanosheet: (d) AFM image of GaAs NS (e) Schematic of device functioning (Inset: Optical image of device), (f) Time-resolved photocurrent of the photodetector in response to light ON/OFF in different gas environments. (d-f) Reproduced from Ref 45 (III) GaSe flake: (g) AFM image of few-layer GaSe flake, (h) Schematic of photodetectors and (i) Photocurrent as function. (gi) Reproduced from Ref 79 (IV) ZnSe nanobelt: (j) SEM image of ZnSe NB, (k) Schematic of single ZnSe nanobelt configured as photodetector (Inset: SEM image of device) and (l) Comparative I–V characteristics of the device illuminated with 400-nm wavelength light and under dark conditions. (j-l) Reproduced from Ref 50

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Figure 6. Peculiar UV Photodetectors- (I) Nb2O5 nanoplate: (a, b) SEM image and pictorial representation of Nb2O5 nanoplate photodetector respectively and (c) Timedependent responses of the photodetector upon illumination of the 320 nm light measured using the mechanical chopping method. (a-c) Reproduced from Ref 140 (II) Nb2O5 nanobelt: (d) Cross-sectional SEM image of Nb2O5 nanobelt arrays, (e) Schematic diagram of single Nb2O5-nanobeltdevice (Inset: SEM image of device) and (f) Photocurrent-time plot. (d-f) Reproduced from Ref 52 (III) In2Ge2O7 nanobelt: (g) SEM image of group of In2Ge2O7 nanobelts, (h) Schematic of the device and (i)Time response of an In2Ge2O7 -nanobelt photodetector. (g-i) Reproduced from Ref 146 (IV) BP flake: (j) AFM scan across the channel area of our device showing the thickness of the black phosphorus flake to be ∼4.5 nm, (k) Three-dimensional view of the BP photo-FET device structure to measure photoresponse and (l) Photoresponsivity of BP device and Combination of fast and slow response of photocurrent with excitation of 390 nm light source at the same applied back gate. (j-l) Reproduced from Ref 147

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Figure 7. CdS nanobelt/nanoribbon Visible Photodetectors- (I) CdS nanobelt: (a) Highmagnification SEM image, (b) Schematic of CdS-nanobelt photodetector and (c) Reproducible on/off switching upon 490nm light illumination. (a-c) Reproduced from Ref 23 (II) CdS nanobelt: (d) FESEM image, (e) Schematic illustration of single CdS NB MESFET based photodetector and (f) On/off photocurrent response of the CdS NB without Schottky contact as a function of time. (d-f) Reproduced from Ref 154 (III) CdS nanoribbon: (g) SEM morphology of the as-synthesized CdS NRs, (h) I-V curves of CdS single nanoribbon illuminated with light of different wavelength. Insets: Optical microscopic image of the single-NR device (left), schematic of the configuration for photoconductive measurements (right) and (i) Time response of CdS NR to the pulsed incidence light (490 nm). (g-i) Reproduced from Ref 46

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Figure 8. Doped CdS and CdSe NR/NB based Visible Photodetectors- (I) Er:CdS nanoribbon: (a) SEM image of single Er:CdS NR at high magnification, (b) Illustration of the detector configuration and (c) I-V curves of Er:CdS NR device under illumination with an incandascent lamp, Inset: Optical microscopic image of a single Er:CdS NR detector. (a-c) Reproduced from Ref 156 (II) Ga:CdS nanoribbon: (d) FESEM image of Ga:CdS NRs, (e) The energy band diagrams of open-circuited and short-circuited Ga:CdS NR/Au Schottky junction device under light illumination and (f) Time response at zero bias under light illumination of 510 nm. (d-f) Reproduced from Ref 157 (III) CdSe nanoribbon: (g) TEM image of individual CdSe NR, (h) Schematic illustration of PPD and (i) Photoresponse of a pure CdSe NR and HGNs@CdSe NR PD under 650 nm light illumination. (g-i) Reproduced from Ref 161 (IV) CdSe nanobelts: (j) FESEM image of CdSe NBs, (k) Schematic diagram of experimental setup adopted for studying the photoresponse speed of CdSe NB PDs and (l) Reproducible light on/off switching properties of the two devices (Inset: natural logarithmic plot of the time response spectrum). (j-l) Reproduced from Ref 60

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Figure 9. GaTe nanosheet Visible Photodetectors- (I) GaTe nanosheets: (a) SEM image of GaTe NS, (b) Schematic of PD (Inset: GaTe NS device), and (c) Time-resolved photoresponse at different bias voltage Vds = 0.1, 0.5 and 1 V. (a-c) Reproduced from Ref 163 (II) Multilayer GaTe: (d) Optical image of multilayer GaTe device (Scale bar: 10 µm), (e) 3D schematic view of multilayer GaTe device (f) Time-resolved photoresponse of the multilayer GaTe device recorded by alternative switching on and off the light illumination. (d-f) Reproduced from Ref 165 (III) GaTe nanosheets: (g) SEM image of GaTe NSs (Inset: Enlarged SEM image), (h) Optical images of GaTe NS device and (i) Time-dependent photoresponse with the laser on and off after bending different times. (g-i) Reproduced from Ref 162

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Figure 10. Metal sulphide nanostructures based Visible Photodetectors- (I) Single-layer MoS2: (a) Optical image of single-layer MoS2 flake and device (b) 3D schematic view of single-layer MoS2 photodetector (c) Cross-sectional view of the structure of the single-layer MoS2 photodetector together with electrical connections. (a-c) Reproduced from Ref 168 (II) Single-layer MoS2: (d) AFM image of single-layer MoS2, (e) Optical image of FET device made by single-layer MoS2and (f) Photoswitching characteristics of single-layer MoS2 phototransistor at different optical power and drain voltage. (d-f) Reproduced from Ref 169 (III) ZrS2 nanobelt: (g) High-magnification SEM image of ZrS2, (h) Schematic of a ZrS2 nanobelt FET device, Inset: SEM image of device and (i) Reproducible on/off switching upon 450-nm light illumination. (g-i) Reproduced from Ref 167

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Figure 11. Nanostructured Visible Photodetectors- (I) GeS nanoribbon: (a) SEM images of GeS NRs, (b) GeS NR photodetector and (c) Time dependence with a bias voltage of 5 V under 530 nm light illumination. (a-c) Reproduced from Ref 174 (II) Mo:ReSe2 nanosheet: (d) AFM image of few-layer Mo:ReSe2 NS, (e) Schematic of the device operation and (f) I-t curves.(d-f) Reproduced from Ref 170 (III) Multi-layer BP: (g) AFM image of BP PD, (h) Schematic of the imaging process and (i) Scanning photocurrent micrographs of the shortcircuit photocurrent excited at λVIS=532 nm with an optical power density of 13.4 kW/cm2. (g-i) Reproduced from Ref 173

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Figure 12. GeSe and graphene nanoribbon based IR Photodetectors- (I) GeSe nanosheets: (a) Low-magnification SEM images of GeSe nanosheets, (b) The performance of GeSe NS-based PD device under 808 nm-light illumination. Inset (top-right) represents the schematic of global irradiation of laser light onto the device during photocurrent measurements. Inset (bottom-right) shows the SEM image of the fabricated GeSe NS device and (c) Time-dependent photocurrent response of the device to laser light illumination with different light intensities under vacuum (4 × 10−3 mbar) at fixed bias of 4 V. Inset: The enlarged views of 32.6−33.4 s range (from light-off to light-on transition) representing response time of ∼0.1 s. (a-c) Reproduced from Ref 178 (II) Graphene nanoribbons:(d) TEM image of GNRs and HRTEM image (inset), (e) Representative diagram for photodetector mechanism and (f) Photocurrent as a function of time with different IR intensities at 2 V for GNRs. Inset: Photoresponse at 80 mWcm− 2. (d-f) Reproduced from Ref 134 (II) Graphene nanoribbons: (g) SEM of close-up of the contact area of GNR array photodetector with 100 nm GNR width and 100 nm GNR spacing (Scale bar 2 mm). The length and width of the entire array are 30 and 10 mm, respectively, (h) Schematic of the photoconductivity setup and (i) Scanning photocurrent image of GNR superlattice photodetector (Scale bar 30 mm). (g-i) Reproduced from Ref 211 References

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Nanoscale Accepted Manuscript

Published on 10 February 2016. Downloaded by University of Massachusetts - Amherst on 11/02/2016 02:27:08.

DOI: 10.1039/C5NR09111J