1. Near-infrared spectroscopy history and principle

Infrared light is a portion between the visible and microwave regions of the electromagnetic spectrum ranging from 700 nm to 1 mm. Infrared light was discovered by a German-British astronomer named William Herschel in 1800 while investigating the temperature difference among the colors in the visible spectrum by using thermometers. He perceived the value of increased temperature in the thermometer scale different from the red light of the visible region. Herschel assigned that region as infrared light and postulated that infrared light can be sensed as heat. Any object with a temperature of >268 °C (450 °F) can emit infrared radiations.

Ever since the NIR spectroscopy was introduced into the American agriculture during the 1950’s when Karl Norris successfully applied this technology to analysis the product, the development of NIR spectroscopy never stops. Benefited from the Chemometries and the great promotion of computing power of Central Processing Unit (CPU), the NIR spectroscopy today is able to provide reliable data analysis.

Near-infrared spectroscopy is based on the principle that every molecule is constructed by several atoms connected with the characteristic bonds between them. When such molecule is excited with a specific radiation of light, it undergoes short-term vibrations in terms of reflection, transmission, and absorption within the molecule based on elemental constituents and bond strength. The nature of such light behavior is unique for each organic molecule and acts as a characteristic spectral fingerprint.

The improper classification of infrared light is more common in practice. However, the InternationalCommission on Illumination classified infrared light into three categories based on photon energy, as presented in Table 1. Alternatively, the International Organization for Standardization 20473 classified infrared light based on its wavelength, as shown in Table 2.

Table 1. The infrared light of the International Commission on Illumination.

Table 2. The infrared light of the International Organization for Standardization 20473.

2. The traditionalInfrared light sources andpc-NIR LED

Traditionally, the commercially available light sources are tungsten halogen lamps, laser diodes, supercontinum lasers and globars. Although they may partially meet the luminescence properties required for laboratory usage, the severe heat generation and incapability of compact design hinders their application towards miniature size or portable spectrometers. One hand, in terms of optical properties, the unstable spectra stability still matters. One the other hand, physical properties such as short lifetime and high-power consumption are in a high demand of promotion. One of the primary issue to bring the laboratory technology in daily life for food analysis and healthcare is to narrow down the massive size of standard desktop laboratory spectrometers into portable size like smartphones, which requires the efforts not only devoted to the light detecting systems but also the NIR light sources. The prevailing and mature technology, solid-state lighting based on InGaN LED chips and phosphor-converted layers is a promising candidate, which meet the demand of miniature size, flexible design, long lifetime and some other physical properties. Now the key point is whether the phosphor-converted LED technology can provide the required optical properties for specified applications.

Phosphor-converted near-infrared light emitting diode (pc-NIR LED) is a promising alternative light source for miniature NIR or portable hand-held spectrometers because of its remarkable advantages of smaller size, longerlifetime, stable spectral stability, and lower cost than traditional light sources. The resulting spectral light distribution from pc-NIR LED light source should be considerably high for effective and efficient functions. Various organic elements present in foodstuffs and human body possess the absorption and reflection spectra of light in the blue region (450～600 nm) and infrared light (700～900 nm) of the electromagnetic spectrum, respectively. Hence, broadband near-infrared phosphors that are excitable by blue light are highly desirable to develop miniature spectrometers.

LED is a semiconductor device constructed by two semiconductor materials, which include both p-type and n-type semiconductors, as shown in Figure 1a. On the application of voltage in the forward bias condition, the electrons from n-side and holes from p-side are recombined in the depletion zone and photons are released. The wavelength of the emitted photons is substantially determined by the use of phosphor conversion materials, and the elements are included in the semiconductor device. The structure of pc-NIR LED optoelectronic component shown in Figure 1b consists of a blue semiconductor chip generally on the basis of aluminum indium gallium phosphide or indium gallium nitride mounted on the epitaxial layer sequence in emitting blue light. The semiconductor chips are connected to the cavity in the base housing through a bonding wire. The base housing is embedded in a recess. The weighted amount of inorganic phosphor conversion materials is mixed with the matrix materials (i.e., silicone) and filled the cavity in the base housing frame. The reflector is mounted on the base housing unit with suitable materials coated on the inner walls of the recess to attain the maximum reflection of primary radiations from the blue semiconductor chip and infrared luminescence from the conversion materials embedded in the matrix materials. During operation, the blue light emitted from the semiconductor device will act as an excitation source of light for the phosphor conversion materials and results in infrared luminescence. The near-infrared LED device appears bluish white in addition to invisible infrared LED during the operation due to the possible combinations of visible red and blue lights. The fabrication of a pc-NIR LED device follows the general principles of white LED devices. In brief, the emitted light from the blue LED chip is used to excite the near-infrared phosphor deposited over the blue-chip, which results in near-infrared luminescence.

Figure 1. (a) The structure of a semiconductor device; (b) The structure of pc-NIR LED optoelectronic component.

3. The types of Cr³⁺-doped near-infrared inorganic phosphor host

The overview of some reported scientific articles shows that the luminescent center for nearinfrared light in the phosphor material can be rare earth elements (i.e., Pr³⁺, Nd³⁺, Tm³⁺, Eu²⁺) or transition metal elements (i.e., Cr³⁺, Ni²⁺, V²⁺, Mn⁴⁺). Among these NIR phosphors, the transition element Cr³⁺ showed higher efficiency andmatched well with blue LED chips. Cr³⁺-doped materials are investigated well by the scientific community for laser and persistent luminescence applications for the past several decades. The phosphor host system for near-infrared light is classified into five types, as shown in Figure 2.

Figure 2. The types of phosphor host system for Cr³⁺-doped near-infrared light.

3.1  Zn-gallogermanate system

The spinel crystal structure of MGa₂O₄ (M = Zn, Mg) is focused on persistent luminescence studies due to the possible generation of antisite defects and O vacancies due to Cr³⁺ transition element doping. Although the emission bandwidth is high (650–770 nm), the presence of antisite defects may deteriorate the fluorescence performance and increase the decay time due to spin-forbidden transition alone, thereby making the Zngallates and -gallogermanate system as inferior candidates for broadband near-infrared spectroscopy applications.

3.2 Ca-gallogermanate system

The Ca-gallogermanate system is another category of a host system for the broadband near-infrared spectroscopy applications. The selection design principle and chemical composition of this system are almost similar to those of the Zn-gallogermanate system, but Ca is present in the host system instead of Zn. Most Ca-gallogermanate compounds are located in the weak crystal field region of the d₃ Tanabe–Sugano diagram due to the presence of large divalent cation in the host. Hence, Cr³⁺ in Ca-gallogermanate compounds follows the spin allowed transitions and results in tunable broadband emission spectrum.

3.3 Garnet-type system

The garnet-type crystal structure with the chemical formula of A₃B₅O₁₂ (A = Y, Gd, La, Lu and B = Al, Ga) is also investigated for Cr³⁺ doping in the B³⁺ site and focused mainly on persistent luminescence studies. Garnet-type persistent luminescent phosphors show uniform single broadband near-infrared luminescence in the range of 650–800 nm centered at 700 nm, which is in contrast to those of Zn-gallate and Zn-gallogermanate systems, and the emission bandwidth of which is extremely narrow for broadband near-infrared spectroscopy applications.

The rigid crystal structure promises the garnet family a good choice served as host material for emitting ions, for example the Y₃Al₅O₁₂:Ce is a superior yellow phosphor with high quantum efficiency and small thermal quenching for white light pc-LEDs. It is also a good choice of Cr³⁺ activated host candidates despite the only one available octahedral site.

3.4 La-gallogermanate system

La-gallogermanate systems are also investigated for the persistent luminescence capability by doping Cr³⁺and rare-earth elements as codopants or sensitizers. The emission spectrum of Cr³⁺-doped LaGaO₃ single crystal reveals several narrow lines with the two maximum positions at 739 and 729 nm. The La-gallogermanate systemmay be a good potential candidate for fluorescence property and is inferior to phosphorescence.

3.5 Borate system

The borate-based host system is also focused on broadband near-infrared spectroscopy applications in recent times. The borate-based host system possesses few thermal quenching behavior, which is one of the main important parameters required for LED semiconductor. Meanwhile, the use of the borate host system eliminates the high-cost starting precursors, such as Ge and rare earths, in the host, which is an additional advantage considering its practical implementations.

4. Known broadband near-infrared phosphors for spectroscopy applications

Near-infrared spectrometers are nondestructive analytical characterization tools using near-infrared light to perform near-infrared spectroscopy function in the diverse applications, including agriculture, pharmaceutical, food industry, and noninvasive health monitoring.

For NIR pc-LEDs, the exclusive merit compared to the semiconductor chips is the broadband luminescence spectrum and the spectral tunability, hence the NIR pc-LEDs is developed fast to accelerate the approaching of NIR spectroscopy into daily life. What is more, the versatile choices of abundant NIR emitting ions provide phosphors that enable the NIR pc-LEDs replacing semiconductor chips, which are designed for special usage that requires determined wavelength.