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(2) Phosphor- or scintillator-based detectors are used as an indirect detection method, which is primarily for X-ray imaging. When excited by radiation energy these materials can generate ultraviolet (UV) to visible light, i.e. fluores-cence. This method of detection is named as indirect detec-tion. The generated light pulse intensity follows an exponen-tial decrease, and the decay time is defined by the time when the light intensity diminishes to 1/e (e is a mathematical con-stant) of its maximum value [3]. In the nonradiative deexci-tation process, quenching occurs if the excitation is de-graded, resulting in reduced light output. Phosphors are used for counting techniques, while scintillators are used for inte-grating techniques [3, 6]. In practice, phosphors are powders that are used on screens for detecting soft X-rays (X-rays having energies below 5−10 kV, or above 0.2−0.1 nm wave-lengths). Scintillators are made as bulk single crystals for detecting hard X-rays (X-rays with energies above 5−10 kV, or below 0.2−0.1 nm wavelengths), or γ-rays. Phosphors consist of a host inorganic matrix that contains luminescent centers at an appropriate doping concentration, typically be-tween 0.01 and 5% [7]. These two terms, phosphor and scin-tillator, are often used interchangeably [6]. Generated visible light can be used for detection using a photomultiplier tube (PMT), photodiode (PD), complementary metal oxide semi-conductor (CMOS), or charge-coupled device (CCD), and thereby, enabling real-time digital imaging.

(3) Semiconductor-based detectors are used as a direct detection method for X-ray imaging and X-ray exposure measurements [8-12]. X-ray photons that are converted to electron-hole pairs, which are subsequently collected to indi-cate interaction of photons within the crystal. This method of detection is named as direct detection. Lithium-doped silicon and germanium, Si(Li) and Ge(Li), respectively, are widely used in analytical instruments as X-ray energy-dispersive spectrometers (EDSs) [13].

(4) Gas detectors are used for X-ray exposure measure-ments. When X-rays travel through a gaseous medium, molecules are ionized, producing positive ions and free elec-trons. When an electric field is applied in the gas chamber, ions and electrons are accelerated in opposite directions. Their collection induces a measureable current [14]. This method is widely used for X-ray exposure measurement, such as gas detectors and wavelength-dispersive spectrome-ters.

With the dramatic developments of nanoscience and nanotechnology, nanomaterials are widely used in many ap-plications. In this review, the materials for the X-ray detec-tions are summarized, and the research on the X-ray detec-tion using nanomaterials is highlighted.

2. MATERIALS FOR X-RAY DETECTION

The materials used for X-ray detection can be classified into the following groups:

(1) Phosphors

Phosphor materials have been used to detect X-rays for a very long time [1, 3]. A typical example is CaWO4, which was the phosphor material used in the discovery of X-rays. Its use for X-ray detection was continued for several dec-

ades. The phosphors are mainly used as powders on screens, converting X-rays into visible light with efficiency typically less than 20% (efficiency of conversion of absorbed X-­‐ray energy to light energy).

The X-ray absorption of a phosphor is determined by the atomic number of the material. The absorption efficiency increases with increasing atomic number. Phosphors are generally spherical particulates. They have a packing density that is significantly less than their single-crystal density. When the coating weight of phosphor per unit area is in the range of 20−50 mg/cm2, the material is well-suited for high-resolution imaging applications (such as digital dental radio-graphy); while in the 50−140 mg/cm2 range, for light-starved lens-coupled systems with lower resolution but a higher light output; and in the very high coating weight of 200−450 mg/cm2 range, for high-energy applications (>450−25,000 keV) [7]. Table 1 lists characteristics of some selected X-ray phosphor materials. Table 1. Selected X-ray phosphor materials [3, 6].

Phosphor Density

(g cm-3)

Decay time (ns)

Efficiency (%)

Emission maximum

(nm)

CaWO4 6.1 6×103 5 420

Gd2O2S:Tb 7.3 6×105 13–16 540

Gd2O2S:Pr,Ce,F 7.3 4000 8–10 490

LaOBr:Tb 6.3 ∼106 19–20 425

Lu2O3:Eu 9.4 ∼106 ∼8 611

SrHfO3:Ce 7.7 40 2–4 390

Y2O3:Eu3+ 5.0−5.5 2,000 8 612

Y2O2S:Eu3+ 5.0−5.5 500 13 Red

YTaO4:Nb 7.5 ∼2,000 11 410

ZnS:Ag 3.9 ∼1,000 17–20 450

(2) Scintillators

Scintillators are historically defined as compounds con-tained within bulk single crystals, which are used to stop high-energy X-rays or γ-rays [3]. They possess high density, short decay time, low production cost, and high radiation hardness (resistance to damage or malfunctions caused by ionizing irradiation). The light yield of scintillators is an ana-logue of the overall efficiency in phosphors. Table 2 lists characteristics of some selected scintillators.

CsI-based scintillators are widely used due to their unique high performance and ease of production. When doped with thallium (Tl), CsI(Tl) crystal scintillators are con-sidered one of the brightest scintillators with a yield of ap-proximately 66,000 optical photons for MeV. CsI(Tl) crystal scintillators have a maximum emission around 550 nm, which is well suited for photodiode readout. They have been widely used for detecting X- and γ-rays. On the other hand, sodium-doped scintillators, CsI(Na), are widely used materi-als that have emission in the blue spectral region; which well

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matches the photocathode sensitivity of bialkali photomulti-plier. It exhibits fast scintillation decay in alpha scintillation, which is a unique property that has not been found in other alkali halides, such as CsI(Tl), NaI(Tl) or pure CsI [17]. Re-cently, CsI(Na) was found to be a promising detection mate-rial for scientific exploration of dark matter in astronomy [18].

It should be mentioned that scintillation materials can be gaseous, liquid, glass-like, organic (plastics), or inorganic; and they are also made as columnar thick film (50–500 µm) or nanostructures [19]. Each variety will be described in Sec-tions 4 and 5. Table 2. Selected X-ray scintillators [1-3, 15, 16].

Phosphor Density (g cm-3)

Light yield (photon MeV-1)

Decay time (ns)

Emission maximum

(nm)

BaF2 4.88 1,500 0.6–0.8 180–220

Bi4Ge3O12 7.1 8,600 300 480

CdWO4 7.9 20,000 5,000 495

CsI 4.51 2,000 16 315

CsI:Na 4.51 40,000 630 420

CsI:Tl 4.51 66,000 800 550

Gd2SiO5:Ce 6.7 8,000 60 420

K2LaI5:Ce 4.4 55,000 24 420

LaBr3:Ce 5.3 61,000 35 358

LaCl3:Ce 3.86 46,000 25 330

LuAlO3:Ce 8.34 12,000 18 365

Lu3Al5O12:Ce 6.67 12,500 55 530

Lu2SiO5:Ce 7.4 26,000 30 390

NaI:Tl 3.67 41,000 230 410

PbWO4 8.28 300 2–3 410

YAlO3:Ce 5.6 21,000 20–30 360

Y3Al5O12:Ce 4.56 24,000 90–120 550

Y2SiO5:Ce 4.45 9,200 42 420

ZnO 5.61 3,200 10 520

(3) Semiconductors

In the event of X-ray irradiation on a semiconductor, X-ray photons are converted to electron−hole pairs, and the number of electrons that can be measured is proportional to the X-ray energy. Typical examples of semiconductors that are widely used as EDSs in analytical electron microscopy are lithium-doped silicon or germanium, Si(Li) and Ge(Li), respectively. Amorphous selenium (Se) is used for practical application in medical imaging, such as imaging detectors for mammogra-phy and chest radiography.

For radiation detection, semiconductors have distinct advantages. Semiconductors have a wide range of stopping power correlated with different atomic numbers, from light elements to heavy elements, as well as, a wide range of band gaps as listed in Table 3. The stopping power and band gap energy can be selected based on specific applications [9]. Table 3. Selected X-ray semiconductor materials [9, 10].

Semiconductor Density

(g cm-3)

Band gap

(eV)

Epair

(eV)

Resistivity

(Ω cm)

4H-SiC 3.17 3.26 7.8 >103

α-Se 4.3 2.1−2.2 45 1014−1015

CdTe 6.2 1.44 4.4 109

Cd0.9Zn0.1Te 5.78 1.57 4.6 1011

GaAs 5.32 1.43 4.2 1011

Ge 5.33 1.57 4.6 50

HgI2 6.4 2.13 4.2 1013

InP 4.78 1.43 4.2 106

PbI2 6.2 2.3–2.6 4.9 1012

PbO 9.6 1.9 8–20 7−10×1012

Si 2.33 1.12 3.6 104

TlBr 7.56 2.68 6.5 1012

3. NANO SIZE EFFECTS

Phosphors or scintillators can be prepared as nanoparti-cles, nanocomposites [20], or as sintered transparent nanoce-ramics [21] based upon the characteristics of the nanoparticles. The advantages and problems of using nanocrystalline scintil-lators were discussed in the literature [22]. When these materi-als are made within the nanoscale, the following nano-size effects are introduced:

(1) Quantum confinement effect. When the size of a par-ticle is decreased to the nanoscale, quantum confinement oc-curs, which is a consequence of the strong reduction of quan-tum mechanical allowed states in a small particle, leading to increased band gap energy [23]. Quantum confinement also modifies the resulting optical constant, luminescence lifetime, and emission quantum efficiency [24]. The relaxation proc-esses of electrons and holes can be affected in small nanopar-ticles. A small shift of the conduction band combined with some shifts of the activator excited states due to crystal field fluctuations may lead to significant changes in the photo-ionization and auto-ionization processes [24].

(2) Surface effect. Nanoparticles possess an increased surface area to volume ratio when compared to their bulk counterparts. Such increased surface area often contains quenching centers, and thus surface passivation is required. One suggested strategy is to produce a core shell system [24]. As schematically shown in Fig. (1), a shell on the nanoparti-cle, either crystalline, amorphous or even molecules could

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Fig. (1). Emission from a single nanoparticle (a) and core/shell struc-tures (b).

produce surface passivation to enhance the luminescence. In LaF3:Eu/LaF3 core/multi-shell nanoparticles, it was found that shelling of the RE-doped core with a non-doped shell increased scintillation intensity by three-fold [25].

(3) Structural effect. When the size of materials is de-creased to the nanoscale, surface tension may lead to structural distortion of lattice parameters [24]. An example of structural effect is highlighted by the fluctuation of Y-O and Zr-O bond-ing distances, as shown in [26]. Additionally, within a small volume of nanoparticles, the crystallinity is improved along with less number of structural defects, compared to the large-volume bulk samples.

4. X-RAY DETECTION USING RANDOM NANOMA-TERIALS

Research on X-ray detection using randomly unaligned nanoparticles, nanowires, nanofibers, or materials based on them are summarized as follows, while the particular applica-tions of vertically aligned crystals are summarized in Section 5.

(1) Nanoparticles

The luminescence yield generally increases when the par-ticle size is decreased, depending on the material [24, 27], which can be a significant benefit when considering practical use of the nanomaterial. Earlier work demonstrated that ce-rium doped Y2SiO5 (YSO) nanoparticles, within a range of 25−100 nm, exhibited enhanced light output, reduced produc-tion cost, and greater size scalability [20, 28]. Fig. (2) shows a comparison of radio luminescence (RL) emission of a nano-phosphor and bulk sample exposed to X-ray excitation, which has been normalized to the sample mass [20]. It shows clearly that the nanopowders exhibit enhanced luminescence intensity over the bulk sample. Klassen et al. found that nanocrystal-line LuBO3 scintillators exhibit enhanced RL, with a maxi-mum intensity at about 90 nm [22]; as the particle size in-creases, the RL intensity decreases. It was then proposed to develop a new generation of devices for radiation detection by controlling the dimensions and structure of nanoscintillator materials [22].

Fig. (2). Comparison of RL of bulk and nano YSO:Ce samples (re-produced from [20] with permission).

Luminescence suppression was also reported. From the photoluminescence (PL) spectra of YAG single crystal, it was found that its intrinsic luminescence was composed of two components from the antisite centers (unwanted) and self-trapped excitation (STE) centers (wanted); while when the sample was in the form of nanoparticles, the intrinsic lumines-cence was considerably suppressed [29]. Such suppression is ascribed to the nanoparticle surfaces that are effective traps for excitons and charge carriers. Using KYF4 nanoparticles with 10−40 nm size doped with Tb3+, it was observed that the in-trinsic cross-luminescence was strongly quenched. This is due to non-radiative energy transfer to crystal lattice defects avail-able in the nanoparticles [30]. A recent work on BaF2 nanopar-ticles demonstrated that when the size of BaF2 nanoparticles is less than 80 nm, these particles exhibit a sharp decrease of STE luminescence intensity, as shown in Fig. (3a). Lumines-cence quenching is caused by electrons escaping from the small nanoparticles [31]. However, when BaF2 nanoparticles are embedded within polystyrene as a nanocomposite scintilla-tor, the luminescence intensity is significantly enhanced by about 15-fold, as shown in Fig. (3b), further confirming the electron escape mechanism. Such a nanocomposite polymer with enhanced luminescence properties is a promising scintil-lator.

Recent research indicated that when the diameter of CsI(Na) particles is decreased to nanoscale, X-ray excited luminescence decay time sped up significantly from ~600 ns to ~10 ns [32]. It was confirmed that diameter reduction led to surface effect, which further accelerated quenching. This type of surface-induced detection method provides a promising prospect for fast X- or γ-ray detections.

(2) Nanofibers and nanowires

By dissolving metal-containing chemicals into polymer solutions at an appropriate ratio, nanofibers can be obtained by high-voltage electrospinning. Fig. (4) shows nanofibers prepared using zinc acetate in polyvinylpyrrolidone (PVP) polymer solution, and after calcination at 500 °C in air, the polymer-based fibers are transformed to ZnO nanofibers. The fiber size is typically reduced from 330 nm to 90 nm after removing the organic scaffold during calcination. By this method, it is convenient to add metal dopants in the

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Fig. (3). X-ray excited luminescence spectra of BaF2 microcrystals and nanoparticles (a); and polymer-matrix nanocomposites with nanoparticles in two different sizes compared with a polymer scin-tillator film (b) (reproduced from [31] with permission).

polymer solution preparation process. Oxide nanofibers of scintillators [33, 34], phosphors [35] and semiconductors [36] have been prepared in recent years. The electrospun Lu2SiO5:Ce3+ nanofibers exhibit a strong emission peak lo-cated at about 403 nm, and a fast decay time of 8.56 ns that was not found in the single crystal [33]. The fast decay time originated from surface and grain boundary effect of poly-crystalline fibers.

Single semiconductor nanowires can be used to detect hard X-rays [37, 38], as the semiconductors can convert X-ray photon energy into an electric signal. A 100 nm InP nanowire was used to directly image an X-ray nanofocus, which achieved submicron resolution [37].

(3) Nanocomposites

Traditional ways to grow single crystal scintillators by us-ing high-temperature furnace produce not only high-cost products, but also possible doping gradients, causing non-uniform light yield that degrades resolution [39]. An alterna-tive strategy to prepare bulk scintillators as nanocomposites or nanoceramics has attracted great attention [40-45]. Sahi et al. synthesized CeF3 nanoparticles of 12 nm size, and dispersed the nanoparticles into polyvinyltoluene (PVT) monomer [40]. The luminescence is enhanced by CeF3 nanoparticles to three and 2.5 times under UV and X-ray excitations, respectively. Lee et al. prepared a BaGdF5:Tb glass ceramic as a nano-composite scintillator for X-ray imaging [41]. Raw materials in a correct ratio were sintered at 1400 °C and then quenched

Fig. (4). (a) Prepared electrospun nanofibers; (b) ZnO nanofibers after calcination.

to obtain a glassy sample. Subsequent annealing at 550−700 °C promoted nucleation and growth of nanocrystals inside the glass matrix. This type of melt-quenching route was also used to prepare Tb3+-doped oxyfluoride scintillating ceramic glass that contained KLu2F7 nanocrystals [42], and oxyfluoride scintillating ceramic glass that contained KTb2F7 nanocrys-tals [43]. Procházková et al. prepared ZnO:Ga and ZnCdO:Ga nanopowders embedded into SiO2 matrix by chemical method [44]. Ning et al. prepared amorphous YAG:Ce nanoparticle-based films by electron beam deposi-tion [45].

5. X-RAY DETECTION USING ALIGNED COLUM-NAR CRYSTALS

For X-ray imaging, in order to improve the spatial reso-lution, scintillators are prepared as vertically aligned col-umns, with or without light guides [4] to reduce cross-scattering of light. As depicted in Fig. (5), in the case of a planar scintillating foil, the spatial resolution is low. The spatial resolution is approximately on the same order as the foil thickness (several hundreds of micrometers), which is due to lateral scattering of photons (Fig. (5a)). If the scintil-lator is confined within narrowed channels along with light guides, the light is restricted to limited areas. Improvements in spatial resolution are inversely proportional to the size of the scintillating crystals (Fig. (5b)). In addition, note that in Fig. (5a), the light signals are spread over a larger area, re-sulting in a lower signal to noise ratio (SNR); while in Fig. (5b), because of the columnar scintillators containing light

6 Current Nanoscience, 2017, Vol. 13, No. 3 Luo et al.

guides, an increased SNR results. Increased SNR allows the detector to achieve greater sensitivity. As pointed out by Badel et al., vertically grown columnar scintillators without light guides could have improved spatial resolution; while they showed low performance in terms of SNR and sensitiv-ity [46]. The scintillator design containing embedded light guides is desirable.

X-ray imaging detectors developed using aligned co-lumnar crystals can be classified into the following catego-ries:

(1) Vertically aligned columnar scintillators without light guides [47-61]. Vieux and Groot described the prepara-tion of needles from alveolate surface of aluminum substrate, and such alveolate surface was prepared by anodization in an electrochemical bath [49]. The anodization resulted in for-mation of anodic aluminum oxide (AAO) nanopores (nano-tubes) on the surface, which facilitated the growth of thinner needles. Nagarkar et al. reported micro-columnar structures resulted from vapor deposition, with thickness ranging from 30−2000 µm and up to 15×15 cm2 in area [51, 52, 56]. Such vertically grown structures could reduce the lateral spreading of light, and therefore improved the detector’s spatial resolu-tion [56]. CsI(Tl) films with columnar structures were depos-ited by thermal evaporation method [59, 60]. Recently, Ko-bayashi et al. prepared (0001)-oriented ZnO nanowires using electrodeposition for X-ray imaging [61]. The diameter was about 40 nm, and length, 200 nm. The nanowire scintillator exhibited improved emission intensity and resolution as compared to ZnO layer scintillator.

Fig. (5). (a) Planar scintillating foil; (b) columnar scintillator with light guides.

(2) Scintillators in microcavities, microchannels, or wells [4, 62-72]. In a report by Rocha et al., a deep reactive-ion etching (DRIE) technique was used to achieve perfect vertical side-walls with 515 µm depth and 100 µm pixel square size [62]. The inner walls of a well were coated with aluminum to improve the number of photons which arrives at the detector and for reducing cross-talk between adjacent wells. Simulations and modeling showed approximately 26% improvement in detection efficiency when using an alumi-num layer [67]. Recently, Yao et al. prepared CsI(Tl) films on specially designed patterned substrates with regular ar-rays of pits [72]. Each CsI(Tl) micro-column were located in one pit. Such CsI(Tl) films had superior performance for X-ray imaging.

(3) Scintillators in aligned nanochannels. Compared to the extensive research on scintillators in microchannels, the research on scintillators in nanochannels with light guides has just started in recent years. Ohashi et al. grew GdAlO3:Ce3+ scintillator fibers surrounded with α-Al2O3 using micro-pulling down method to get directionally solidi-fied eutectics [73]. Scintillator fibers converted X-rays to lights, which were confined and transported along the fiber directions. Light scattering was reduced to improve the X-ray imaging resolution, which almost reached the resolution limit of the material.

Taheri et al. theoretically simulated X-ray detection by ZnO nanowires in AAO [74], ZnO in polycarbonate mem-brane [75], and GaN in AAO [76], respectively, by using the Geant4 Monte Carlo code. According to their simulation results on 200-nm-diameter ZnO nanowires in AAO, it was predicted that the AAO template has a special impact as a light guide to conduct the X-ray-induced photons toward the detector, as well as, to decrease the light scattering in the detector, which resulted in improved spatial resolution in scintillator-based X-ray imaging systems, especially for medical applications [74]. In the parameters used for simula-tion, the light output was increased 20−160%, while the de-tection efficiency was reduced due to the loss of photons, suggesting that future optimization is needed. In another simulation of ZnO nanowires embedded in polycarbonate membrane [75], it was found that the nanowire-based X-ray imager can improve the quality of the output image and the spatial resolution, compared to the traditional imager without nanowires. The typical full width at half maximum of nanowire-based imager in the simulation is five times smaller than the crystalline detector. The simulation results for GaN in AAO [76] indicated that the optimum dimensions of the GaN nanowire for the purpose of enhancing spatial resolution were 10 µm in length and 150−200 nm in radius. Based on the simulation of a scintillating screen containing 2,000,000 nanowires, the spatial resolution of such a detector is less than 1 µm, with an efficiency of approximately 17%.

Chen et al. experimentally prepared CsI nanowires in AAO [77, 78], by a mechanical injection method [79, 80]. In a home-made injection device [79, 80], CsI was placed on the top of AAO template. Upon heating to a temperature above the CsI melting point of 621 °C, a strong pressure is applied to inject melted CsI into the AAO nanochannels. The device is filled with Ar to avoid oxidation of CsI. Fig. (6) shows the CsI nanowires filled inside AAO. Highly ordered

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and vertically aligned CsI nanowires were obtained, with almost full filling rate inside the AAO nanochannels. The diameter of the nanowires depends on the AAO size, ranging from 100-450 nm.

Recently, Taheri et al. fabricated an X-ray imager based on ZnO scintillator nanowires by electrodeposition in porous polycarbonate membrane with a pore size of 200 nm [81]. The ZnO nanowires acted as scintillating fibers in the pres-ence of photons and prevented other generated optical pho-tons from spreading inside the detector. Detection quantum efficiency reached 11%, and spatial resolution was better than 6.8 µm. Zhang et al. recently prepared Lu3Al5O12:Ce3+ nanowire arrays using AAO templates by a sol-gel method [82]. The prepared nanowire arrays exhibited PL and RL emissions.

Fig. (6). CsI crystals in AAO along (a) bottom and (b) cross-sectional views (reproduced from [78] with permission).

CONCLUSION

Currently, solid materials consisting of phosphors, scintil-lators and semiconductors are used to detect X-rays. Enhanced luminescence and shortened decay time were found by using scintillator and phosphor nanoparticles, while luminescence suppression was also observed. Assembly of a core/shell struc-ture was suggested as a general strategy to control the lumi-nescence, as well as, to get increased intensities. Nanofibers and nanowires have been prepared for improved spatial reso-lution. Scintillators are historically made as large single crys-

tals by using complex procedures, while nanocomposites with scintillator or phosphor nanoparticles have been developed at reduced cost while exhibiting high performance. To achieve high spatial resolution in X-ray imaging, it is preferable to vertically align the scintillator materials to reduce light cross scattering. In recent years, scintillators confined to nanochan-nels have been made for improved spatial resolution and light output.

Overall, the application of nanomaterials for X-ray de-tection is limited when compared to other applications using nanomaterials. The research on X-ray detection using nano-materials is an open field for conducting studies of excitation mechanisms, exploration of new scintillator materials, and applications of active compounds.

CONFLICT OF INTEREST The authors confirm that the information contained within this article has no conflict of interest.

ACKNOWLEDGEMENTS The authors acknowledge funding from the NSF No. 1436120 for this research. The instrumentation at Fayetteville State University was supported by the Army Research Office (ARO), Department of Defense (DoD), un-der contracts of W911NF-09-1-0011, W911NF-14-1-0060 and W911NF-15-1-0566. The authors thank Dr. Jason Davis, Oak Ridge Associated Universities (ORAU), for reading and correction of the manuscript. Critical comments from six independent reviewers are greatly appreciated.

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