The present invention relates generally to a system and method for light delivery to a sample subject to examination with a transmission electron microscope (TEM) and, in particular, to a fiber-optic-based system and method of irradiation of such sample with light configured to not interrupt auxiliary functions and capabilities of an environmental TEM that are used during the interrogation of the sample with the TEM.
Photocatalysis is a field of energy research that currently attracts a lot of attention and requires to quantitatively connect structure and processing of catalyst material with their properties and performance, both of which should preferably be enhanced. Often the structural features involved in catalytic processes are at the nano-scale or below. Currently available techniques used for characterization of the catalyst materials, including the characterization with a transmission electron microscopy, would benefit from being enhanced. For example, enhancing current environmental TEM studies of a photocatalyst sample (such as titania or TiO2, for example) by providing high intensity visible and UV illumination of the sample within an environmental transmission electron microscope (ETEM) may allow the user to analyze, in real time, the interaction between light and a photocatalyst under reaction conditions.
Embodiments of the invention provide a transmission-electron microscope (TEM) system for interrogation of a photocatalyst sample. Such TEM system includes a TEM chamber containing an electron gun and an electron lens such that the electron gun and electron lens define a first direction of propagation of an electron beam. The TEM system further includes a holder configured to support the photocatalyst sample a surface of which is positioned substantially perpendicularly to the first direction; and a fiber-optic component having an axis extended in a second direction that is substantially transverse to the first direction. The fiber-optic component is configured to deliver irradiating light from outside of the TEM chamber towards the photocatalyst sample and irradiate the photocatalyst sample with a beam of so delivered irradiating light at an angle of about 70 degrees or less as measured between an axis of said beam of light and a normal to the surface of the photocatalyst sample. In one embodiment, the fiber-optic component includes a multimode quartz optical fiber defining an output facet of the fiber-optic component at an acute angle with respect to the axis. In a specific embodiment, the acute angle is about 60 degrees. The TEM system may further include a light source adapted to generate the irradiating light characterized by a broad-band optical spectrum and disposed externally with respect to the TEM chamber, and at least one of a reflective optical component positioned to focus the irradiating light on an input facet of the fiber optic component and an optical filter disposed such as to transmit the irradiating light therethrough. In a specific embodiment, the TEM system optionally further includes an auxiliary device operably associated with the holder and configured to facilitate one or more of introduction of gas to the surface of the photocatalyst sample and changing a temperature of the photocatalyst sample during an interrogation of the photocatalyst sample with the electron beam. Here, the fiber-optic component is disposed such that an operation of the auxiliary device remains uninterrupted during such interrogation.
Embodiments of the invention further provide a method for interrogation of a photocatalyst sample with a transmission electron microscope (TEM) system. The TEM system includes a TEM chamber containing a sample holder, an electron gun and an electron lens. The TEM system additionally includes at least one auxiliary device operably associated with the holder and adapted to predeterminably achieve at least one of a reactive gas environment around the sample and variation of a temperature of the sample. The electron gun and electron lens define a first direction corresponding to propagation of an electron beam. The method of interrogation includes (i) positioning the photocatalyst sample in the TEM chamber, with a surface of the sample being substantially perpendicular to the electron beam; (ii) transmitting the electron beam through the photocatalyst sample to form an image associated with the sample on a TEM screen; and (iii) irradiating a surface of the photocatalyst sample with light delivered from outside of the TEM chamber through a fiber-optic component that has an axis extending in a second direction. The second direction is generally substantially transverse to the first direction. The irradiance of the irradiating light on the incident surface of the sample is at least 1,000 mW/cm2. In addition, the method may include activating such auxiliary device such to establish interaction between the reactive gas and the surface of the photocatalyst sample and/or change the temperature the photocatalyst sample such that the process of irradiation of the surface of the sample with light delivered through the fiber-optic component does not impede either of the process of interaction and changing the temperature. In a specific implementation, the process of irradiation of the sample includes illuminating the surface of the photocatalyst sample with delivered light at an angle of about 70 degrees or less (as measured between an axis of said beam of light and a normal to the surface of the photocatalyst sample) through an output facet of the fiber-optic component, where the output facet is formed at an acute angle with respect to the optical axis of the fiber-optic component
The invention will be more fully understood by referring to the following Detailed Description in conjunction with the Drawings, of which:
The transmission electron microscope (or TEM) is an optical analogue to the conventional light microscope. Its operation is based on the fact that electrons can be ascribed a wavelength (of the order of about 2.5 pm) and interact with magnetic fields as a point charge. In operation, a beam of electrons is applied instead of a beam of light, and glass lenses are replaced by magnetic lenses. The lateral resolution of the best TEMs is comparable with atomic resolution. A schematic presentation of the microscope is shown in
In-situ TEM encompasses a broad range of techniques which attempt to couple various stimuli of the TEM sample with high resolution imaging. One stimulus which is of interest currently is irradiation of sample with light. In a specific case, when the SUT is a photocatalyst, such irradiation provides a near-reaction conditions and the results of the study may facilitate design of efficient photocatalysts based on understanding of a link between the catalyst microstructure and interaction with light. Optical irradiation of other materials for other characterization purposes may also be carried out (for example, to assess luminescence properties of as a function of local position or composition of a structure). An environmental TEM (ETEM) can be used to study catalysts in situ under conditions of high temperature or in a reactive gas environment. However, the light irradiation experienced by a photocatalyst during use is rarely reproduced in situ, leaving absent a critical experimental condition for studying light-induced processes. According to an embodiment of the invention, an ETEM is adapted for observing the structure, of a photocatalyst sample irradiated with UV and/or visible light, at a scale relevant to catalytic activity. A material absorbing visible and/or UV light is changed slightly by such absorption. For example, chemical reactions can be driven which change the structure or composition of a material, electrons may be promoted to the conduction band in semiconductors, or excited in metals, and these events may become observable. One of the applications of such study is formation of novel nanostructured material for solar energy conversion.
According to the idea of the invention, a system of delivery of light from an external broad-band source to the SUT within the TEM chamber is adapted such that operation of auxiliary element(s) and sub-system(s) of the TEM system that change the environment around the SUT remain operational even when the SUT is irradiated with light. Light-delivery systems of related art possess a shortcoming in that the presence of such light-delivery systems in the TEM chamber impedes the operation of the auxiliary sub-systems (such as sub-systems providing cooling or heating of the sample, or sub-systems adapted to introduce a reactive gas into the chamber). Light has to be delivered to a surface of the SUT at a substantially small angle (as measured with respect to the normal to the surface of the SUT). The presence of optical system(s) inside the TEM chamber has been reported to interfere with the operation of the auxiliary system, for example by blocking their operation during a period of light delivery. (In such a case, simultaneous light delivery and, for example, introduction of the reactive gas in the chamber becomes problematic.)
References throughout this specification to “one embodiment,” “an embodiment,” “a related embodiment,” or similar language mean that a particular feature, structure, or characteristic described in connection with the referred to “embodiment” is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment,” “in an embodiment,” and similar language throughout this specification may, but do not necessarily, all refer to the same embodiment. It is to be understood that no portion of disclosure, taken on its own and/or in reference to a figure, is intended to provide a complete description of all features of the invention.
In addition, in drawings, with reference to which the following disclosure may describe features of the invention, like numbers represent the same or similar elements wherever possible. In the drawings, the depicted structural elements are generally not to scale, and certain components are enlarged relative to the other components for purposes of emphasis and understanding. It is to be understood that no single drawing is intended to support a complete description of all features of the invention. In other words, a given drawing is generally descriptive of only some, and not all, features of the invention. A given drawing and an associated portion of the disclosure containing a description referencing such drawing do not, generally, contain all elements of a particular view or all features that can be presented is this view in order to simplify the given drawing and the discussion, and to direct the discussion to particular elements that are featured in this drawing.
A skilled artisan will recognize that the invention may possibly be practiced without one or more of the specific features, elements, components, structures, details, or characteristics, or with the use of other methods, components, materials, and so forth. Therefore, although a particular detail of an embodiment of the invention may not be necessarily shown in each and every drawing describing such embodiment, the presence of this detail in the drawing may be implied unless the context of the description requires otherwise. In other instances, well known structures, details, materials, or operations may be not shown in a given drawing or described in detail to avoid obscuring aspects of an embodiment of the invention that are being discussed. Furthermore, the described features, structures, or characteristics of the invention may be combined in any suitable manner in one or more embodiments.
Moreover, if the schematic flow chart diagram is included, it is generally set forth as a logical flow-chart diagram. As such, the depicted order and labeled steps of the logical flow are indicative of one embodiment of the presented method. Other steps and methods may be conceived that are equivalent in function, logic, or effect to one or more steps, or portions thereof, of the illustrated method. Additionally, the format and symbols employed are provided to explain the logical steps of the method and are understood not to limit the scope of the method. Although various arrow types and line types may be employed in the flow-chart diagrams, they are understood not to limit the scope of the corresponding method. Indeed, some arrows or other connectors may be used to indicate only the logical flow of the method. For instance, an arrow may indicate a waiting or monitoring period of unspecified duration between enumerated steps of the depicted method. Without loss of generality, the order in which processing steps or particular methods occur may or may not strictly adhere to the order of the corresponding steps shown.
The invention as recited in the appended claims is intended to be assessed in light of the disclosure as a whole.
The guiding of light to the sample area of the TEM with the use of an optical fiber can be arranged by threading the optical fiber along the sample rod, or by introducing the fiber into the chamber through a port on the side of the column. Using a specially designed sample rod is rather problematic, especially if the user of the microscope wants to simultaneously use other commercially available holders to introduce another capability to the microscope, such as heating, cooling, or electrical/mechanical measurements/stimuli. In reference to a diagram of
Additional geometrical and/or space limitations may be introduced by the use of an individual holder. In further reference to
One consideration defining the configuration of a system of light delivery to the sample is the spatial distribution of light at the sample. Once such distribution is characterized outside of the TEM chamber and the amount of light coupled into the fiber is known, the irradiance of light striking the sample in a region of interest defined by position of the electron beam at the sample can be determined. It is desirable for the light delivered to the sample to be confined to a substantially to a small spot on the sample so that high intensities (generally, over 1,000 mW/cm2) are achieved, even at moderate beam power levels. Accordingly the use of a high brightness source of light is necessitated. The high intensities produced in this way are useful for studying materials under a variety of illumination conditions, and allow for studies of solar energy materials under conditions similar to concentrated sunlight schemes.
Defining the spectral distribution of the source of light is also essential. For some solar applications, for example, it may be advantageous to utilize a broadband light source having a spectrum that closely resembles the spectrum of the sun. For other applications, and for many fundamental experiments, it is useful to use a narrow range of incident wavelengths/energies in order to isolate the effect of photons of a particular energy (for example, above or below the bandgap of a semiconductor sample) on such semiconductor sample. The available spectrum should preferably range from the UV (for example, from about 200 nm) up through the entire visible spectrum. Optionally, optical filters can be used to define the spectrum of light delivered to the sample from the spectrum of light produced by the source of light. In some applications, a light source may include a laser or a semiconductor laser (optionally, a wavelength tunable laser). In other applications, the light source may include a superluminescent light emitting diode. Generally, a light source of choice should meet certain criteria impose by the intended application such as, for example, have specific value of brightness or be amenable to be represented, in approximation, as a point source.
Additional considerations dictate that an embodiment of the invention be configured to ensure that the output tip and/or facet of the fiber in the proximity of the sample is repositionable and/or spatially realignable, and that the spatial resolution of characterization of the photocatalyst sample with the electron beam remain no less than about 0.14 nm. Such performance should be satisfied with or without presence of a reactive gas in the TEM chamber (up to a few torr of pressure) and with or without heating of the sample up to about 500° C., while the level of irradiance of light (delivered to the surface of the sample at an angle of at least 45 degrees with respect to the direction of propagation of the electron beam or, alternatively, with respect to the normal to the sample's surface) at least of about 2 to 3 mWcm−2 nm−1 in the visible range and at least 1 mWcm−2 nm−1 in the UV range. These values exceed the AM 1.5 solar spectral irradiance that peaks at about 0.14 mWcm−2 nm−1 in the visible range.
According to an embodiment of the invention, illustrated in
In reference to
In one embodiment, two optical fibers are employed. One is a 2 m long, 600 micrometers in diameter silica core silica clad high OH solarization resistant fiber assembly (from Ocean Optics) that runs from the focal point of the second mirror to the fiber feed through flange which allows light to pass into the fiber on the high vacuum of the microscope. In reference to
An optical fiber used for light delivery according to an embodiment of the invention has a critical bend radius. When bent at a radius smaller than the critical bend radius, such fiber starts leaking light thereby increasing optical losses. Because of this restriction, as well as the limited space in the microscope chamber, the angle that the output facet (cut substantially perpendicularly to the axis of the fiber) can form with respect to the surface of the sample to be irradiated is limited to a maximum angle A in the xz-plane (as shown, at about 15 degrees with respect to the surface of the sample). The curve along which the optical fiber 704 is bent inside the TEM chamber, as shown in
One end of the optical fiber 704 is terminated by a standard SMA connector 720, which connects to the high vacuum side of the feed through flange; the fiber output end 730 near the sample is cut at about 60 degree angle with respect to the fiber axes (shown, indirectly, via an angle B defined with respect to a normal to the fiber axis) to form an output facet correspondingly inclined. This cut provides additional advantages in that it facilitates the delivery of output (irradiating) light 740c to the sample center at an angle that is maximized with respect to the direction of the electron beam (or to the direction defined by a normal to the sample's surface). As a result, the delivered light 740 is not blocked by the walls of the heating holder used in the TEM chamber.
In further reference to
The operation of the embodiment of
Evaluation of the performance of the TEM microscope itself during the process of irradiation of the sample 320 with light, according to an embodiment of the invention, showed that the performance of the TEM was not degraded noticeably by the addition of the fiber illumination system.
Illuminating the TEM sample is essential for studying nanostructured photocatalysts at the smallest length scales in an environment similar to the one they will experience in actual use. Many factors must be considered when designing a system for performing such illumination. The design just described successfully balances these various considerations, and has been shown to not have a significantly detrimental effect on the performance of the microscope, while illuminating the sample with over about 1 W/cm2 of broadband UV and visible light. The opportunity now exists to perform many novel in-situ experiments utilizing light illumination, temperature control, and reactive gas atmospheres, which may provide interesting results for nanostructured photocatalyst materials.
At least some of embodiments of the invention may include the use of a processor controlled by instructions stored in a tangible, non-transitory memory. The memory may be random access memory (RAM), read-only memory (ROM), flash memory, a device readable by a computer I/O attachment, such as CD-ROM or DVD disks, for example), information alterably stored on writable storage media (e.g. floppy disks, removable flash memory and hard drives) or information conveyed to a computer through communication media, including wired or wireless computer networks. In addition, while the invention may be embodied in software, the functions necessary to implement the invention may optionally or alternatively be embodied in part or in whole using firmware and/or hardware components, such as combinatorial logic, Application Specific Integrated Circuits (ASICs), Field-Programmable Gate Arrays (FPGAs) or other hardware or some combination of hardware, software and/or firmware components.
Modifications to, and variations of, the illustrated embodiments may be made without departing from the inventive concepts disclosed herein. Furthermore, disclosed aspects, or portions of these aspects, may be combined in ways not listed above. Accordingly, the invention should not be viewed as being limited to the disclosed embodiment(s).
The present application claims benefit of and priority from the U.S. provisional patent application No. 61/680,078 filed on Aug. 6, 2012 and titled “System and method for irradiating an ETEM-sample with light”. The disclosure of the above-identified provisional patent application is incorporated herein by reference in its entirety.
This invention was made with government support under DE-SC0004954 awarded by the Department of Energy. The government has certain rights in the invention.
Number | Date | Country | |
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61680078 | Aug 2012 | US |